Economic Geology

1. PROCESSES OF FORMATION OF ORE DEPOSITS
Submitted by
Mimag Medak (Roll no. 304)
Kaijirsong Rongpi (Roll no. 377)
Rahul Saikia (Roll no. 239)
Riya Sarmah (Roll no. 03)
Saanju Pegu (Roll no. 280)
Santanu Boruah (Roll no. 199)

2. CONTENTS
◦ Introduction
◦ Types of processes-
a) Magmatic process- 1. Magmatic concentration
2. Hydrothermal process
3. Sublimation process
b) Sedimentary process- 1.Sedimentary deposit
2.Oxidation and supergene enrichment
3. Residual and mechanical concentration process
4. Volcanogenic and supergene enrichment
5. Evaporation process
6. Bacteriogenic process
c) Metamorphic process
Conclusion

3. How do we describe Mineral ?
◦ A mineral is a naturally occurring organic or inorganic
substance, with a definite chemical composition, and
an ordered atomic arrangement.
◦ Economic mineral: This includes those minerals
having some economic value and which are essential
for the economic growth of a country and for
industrialisation. E.g. Iron, gold, aluminium.
They occur as solid masses, or rocks of which the
earth's crust is composed, or as local deposits of
varying size, such as veins, pockets or
impregnations in rocks
The processes of formation of mineral deposits are grouped
into three main types:
(A) Magmatic-1. Magmatic concentration
2. Hydrothermal process
3. Sublimation process
(B) Sedimentary- 1.Sedimentary deposit
2.Oxidation and supergene enrichment
3. Residual and mechanical concentration
process
4. Volcanogenic and supergene enrichment
5. Evaporation process
6. Bacteriogenic
(C) Metamorphic.

4. MAGMATIC ORE DEPOSITS:
https://steemit.com/geology/@bikrambaruah/ore-genesis-1
◦ They are formed by simple crystallization or from concentration by differentiation of intrusive igneous masses.
◦ They have close relationship with the intermediate or deep-seated intrusive igneous rocks.
The Magmatic process of formation of mineral deposits include the following processes:
1. Magmatic concentration. 2. Hydrothermal process 3. Sublimation.

5. 1. MAGMATIC CONCENTRATION PROCESS
• Magma is a mixture of molten or semi-molten rocks, volatiles and solids that is found beneath the surface of the Earth.
Rocks as we know are the aggregates of minerals.
• As the magma approaches the earth’s surface it temperature and pressure drop with the result of crystallization and
differentiation of the different minerals in it in a definite sequence.
• These differentiated minerals collectively forms the magmatic deposits.
• There are different modes of formation of magmatic deposits and they originate during different periods of magma
crystallization. In some the ore minerals crystallize earlier than the host rock whereas in others they remain immiscible
until after the crystallization of the host rock

6. EARLY MAGMATIC DEPOSIT
DISSEMINATION SEGREGATION INJECTION
MAGMATIC ORE DEPOSITS
EARLY MAGMATIC DEPOSITS LATE MAGMATIC DEPOSITS

7. DISSEMINATED ORE DEPOSIT
Dissemination- This are formed by crystallization without consolidation and the ore minerals occur in the form of grains
in various parts of the host rock. Example of such ore deposit include the UraniumdepositsintheSinghbhum granites,Jaduguda,
Bihar
https://www.researchgate.net/figure/A-Disseminated-sulfide-mineralization-in-troctolitic-plagioclase-olivine-rocks-from_fig3_283023045
Fig:Disseminatedores

8. SEGREGATED DEPOSITS
Due to high specific gravity the mafic and ultramafic ferromagnesian crystals separate out from the main magma melt leading
to the formation of segregated deposits. Example: The Chromite-deposit around Jojohatu, Singhbhum district, Jharkhand.
https://www.slideshare.net/saviour123/ugrc-140-earth-resources
The dense minerals like chromite or magnetite settle out of crystallizing magma which you have seen here, being at the
bottom.

9. INJECTED ORE DEPOSIT
The ore minerals are injected into the host rocks or surrounding rocks. Example Vanidiferous magnetite deposits od Dublabera
Singbhum dist.
https://sites.google.com/site/missiontomarsatvssec/home/mission-background-briefing-students/earth/10-igneous-rocks

10. LATE MAGMATIC DEPOSIT
These are formed after the formation of the rock forming minerals at a comparatively lower temperature than the early magmatic deposits.
These are of the following types:
1. Residual liquid segregation: Sometimes in certain basic magmas the residual magma becomes enriched in Fe, Ti etc and in
many Ti bearing magnetite deposit the Fe oxide crystallize last and cut across the earlier formed rock.
2. Residual liquid injection: Here the magmatic melt squirt out from the parent magma into portions of the host rock or
adjacent rocks.
3. Immiscible liquid segregation : Sulphides of Fe-Ni-Cu are not 100% soluble in basic magmas and upon cooling they
separate out as immiscible liquids(fig. below)
4. Immiscible liquid injection: The above mentioned Sulphide rich fraction if subjected to disturbance before consolidation
then it might inject into places of less pressure such as sheared and brecciated areas.
Fig: Immiscible liquid segregation containing Ni-Cu sulphides, S. Africa

11. 2. HYDROTHERMAL PROCESS
◦ It is a part of magmatic end product where magmatic
differentiation may give rise to the concentration of some metal.
◦ Based on temperature of formation it may be divided into:-
Hypothermal, Mesothermal, Epithermal , Telethermal and
Xenothermal deposits
◦ Based on the mode of formation it may be divided into:- Cavity
filling deposit and Replacement deposit.
Cavity filling deposit:-
◦ It consists in deposition of minerals in rock opening from
aqueous solution.
◦ The minerals usually grow inward with development of crystal
faces pointed towards the supplying solution in the form of
comb structure.
◦ Successive crusts of different minerals may precipitated upon the
first one until filling is complete, and finally give rise to
Crustification.
https://www.researchgate.net/figure/Comb-
Structure-in-Secondary-filling-Crystals-in-a-vein-
grow-perpendicular-to-vein-wall_fig9_306263635
Fig: Comb Structure in Quartz

12. VEIN TYPE DEPOSIT
◦ Most vein deposits occur in fault or
fissure openings or in shear zones
within the country rock.
◦ Many of the most productive deposits
of gold, silver, copper, lead, zinc and
mercury occur as hydrothermal vein
deposits
Fissure vein deposit
◦ It is a tabular ore body that contains
one or more fissures.
◦ They may be simple, sheeted, linked,
dilated and chambered
Fig: Simple fissure veinFig: Sheeted vein deposit
Fig: Dilated fissure vein deposit Fig: Chambered fissure vein deposit

13. METASOAMATIC REPLACEMENT
 It is a process of simultaneous solution and deposition whereby one mineral replaces another.
 Replacement occurs when a mineralizing solution encounters minerals unstable in its presence. The original
mineral is dissolved and almost simultaneously exchanged for another.
The replacement may be of different types:
 Massive
 Replacement lodes
 Disseminated

14. 1. Massive
◦ The massive deposits are of irregular form and of
varied size.
◦ It may thicken and thin with wavy outline and ramify
irregularly in all directions
2. Disseminated
◦ The disseminated deposits represent multiple centers
of replacement and consist of altered host rock and
disseminated ore grains.
3. Replacement lodes
◦ The replacement lodes may be massive or high
grade ore flanked by a fringe of disseminated ore
Massive deposit Disseminated deposit
Replacement lodes

15. 3. SUBLIMAATION
This is a minor process in the
formation of mineral deposit.
This process applies to those minerals
that are volatilised and subsequently
redeposited from vapour at lower
temperature or pressure. This are mainly
deposited around volcanoes and
fumaroles but are rarely found in
workable amounts.
Common sublimates include sulphur,
chlorides of iron, copper, zinc, oxides of
iron and copper and various salts of
alkali metals and ammonium.

16. OXIDATION AND SUPERGENE ENRICHMENT
 When ore deposits are exposed to the oxidation zone they
are weathered and altered with the country rocks.
 An ore body thus becomes oxidized and generally leached
of many of its valuable materials down to the groundwater
table, or to depth where oxidation cannot take place.
 As the cold, dilute, leaching solutions trickle downwards ,
their metallic content may be precipitated in the form of
secondary sulfides to give rise to a zone of secondary or
supergene sulfide enrichment.
 The lower unefffected part of the deposit is called primary
deposit.
https://depositonce.tu-
berlin.de/bitstream/11303/1547/1/Dokument_7.pdf

17. RESIDUAL AND MECHANICAL CONCENTRATION
Residual ore deposits are formed by accumulation of valuable minerals when undesired constituents of rocks or mineral
deposits are removed during weathering.
Few requirements for residual concentration :
 Presence of rocks containing valuable minerals of which the undesired substances are soluble and the desired substance are
insoluble under surface conditions.
 Favourable climatic condition for chemical decay.
 Low relief so that the valuable minerals may not be washed away.
 Crustal stability so that residues may accumulate in quantity and deposits are not destroyed by erosion.
https://docplayer.net/50442635-Introduction-to-sedimentary-ore-deposits-hassan-z-harraz.html

18. MECHANICAL CONCENTRATION
It is the natural gravity separation of the heavy minerals from the lighter ones by means of moving water or air. The
concentration of the heavy minerals is termed as placer deposit.
It involves two steps-
 Freeing of the minerals by weathering from their matrix
 The concentration of these minerals. This can occur only if the valuable minerals posses the three properties- high specific
gravity, chemical resistance to weathering, durability.

19. TYPES OF PLACER DEPOSITS
◦ Alluvial or stream placer: These are fluviatile in
origin and located along the inner curves of meanders
of a river. E.g Gold deposits of Subansiri of Upper
Assam.
◦ Eluvial placers: This type of placers occur in down
hill when materials are released from weathered lodes
above the hill. The heavier resistant minerals collect
downslope and the lighter materials are washed away.
◦ Beach placers: These are formed along the sea
shores by concentration effects of wave and shore
action. The shore currents shift materials along shore
and the lighter materials are moved faster than the
heavy, thereby concentrating the heavy minerals.
◦ Eolian placers: In arid regions wind may act as a
medium of concentration of valuable minerals like
gold. The light decomposed minerals are blown away
and the heavy gold particles are left behind freed from
their matrix.
https://www.911metallurgist.com/blog/list-types-placers
Fig: Types of Placer deposits

20. BACTERIOGENIC PROCESS
The metabolic activities of bacteria lead to several chemical and biochemical reactions which in turn enhance the concentration of
certain valuable elements like Fe,Mn,S,As,P which occur as oxides, sulphides and carbonates, which form minerals like goethite, pyrite,
siderite etc.
There are two modes of formation – by oxidation and by reduction.
Bacteriogenic oxidation minerals- These are formed when bacteria oxidises organic matter in the soil. Ex.- Bacteriogenic Iron
Oxides(BIOS) formed by iron oxidising bacteria (FeOB)
Bacteriogenic reduction deposits- These are formed by the reducing action of anerobic bacteria on organic matter. This process is
believed to paly a major role in the formation of BIF
Iron reducing bacteria can reduce insoluble ferric oxide in water containing to soluble ferrous hydroxide and release oxygen. When the
de-oxygenated water reaches a source of oxygen, iron-oxidizing bacteria use that oxygen to convert the soluble ferrous iron back into
an insoluble reddish precipitate of ferric iron.
H2O + Fe2O3 → 2Fe(OH)2 + O2
(water) + (Iron[III] oxide) → (Iron[II] hydroxide) + (oxygen)
2Fe(OH)2 + O2 → H2O + Fe2O3
(Iron[II] hydroxide) + (oxygen) → (water) + (Iron[III] oxide)

21. SEDIMENTARY DEPOSIT
The sedimentary ores are classified as chemical precipitates or mechanical accumulations. Sedimentary ores are syngenetic
deposits. The formation of sedimentary deposit involve-
◦ An adequate source of materials
◦ The gathering of the materials by solutions or other process
◦ The transportation of the materials to the site of accumulation
◦ Compaction , chemical alteration and other changes
The chemical precipitates of sediments is controlled by factors such as pH, Eh of the environment. Certain species of bacteria
can cause the precipitation of oxide compounds by acting as a catalyst.
The process of sedimentary deposit can be can be summarized as-
 During weathering, the materials are released from the source rock. In this process the valuable mineral constituents are taken
into solution. The chief solvents are carbonate water, organic acids and sulphate solutions.
 The valuable substances are transported either in suspension or in solution by means of river to the sea.
 In sea these valuable minerals are deposited mechanically, chemically or biochemically.

22. PRECIPITATIONA OF SEDIMENTARY MANGANESE ORE
In this process iron must be separated from manganese during its deposition. This separation takes place as-
1. Precipitation from carbonate solutions: Manganese carbonate is more stable in solution than iron carbonate, so
manganese carbonate is carried further away and is thus separated.
2. Precipitation in Oxidising Environment: At any given pH iron oxides precipitates at lower Eh than manganese oxide.
Similarly at a fixed Eh, iron oxides precipitate at a lower pH than manganese oxide.
In this manner iron is separated from manganese and is transported further by water and deposited elsewhere.

23. VOLCANOGENIC MASSIVE SULFIDE(VMS) DEPOSIT
◦ The term sedimentary volcanogenic-
exhalative refers to mineral deposit that
have been formed by volcanic
processes and activities of thermal
springs beneath bodies of water.
◦ They are predominantly accumulations
of sulfide minerals that precipitate
from hydrothermal fluids on or below
the seafloor
◦ SEDEX is a kind of VMS but the
difference is that it does not involve
igneous intrusion but are instead
products of thick piles of accumulated
sediments in the ocean. E.g Lead-Zinc
deposit of Agucha, Rajasthan. https://www.911metallurgist.com/blog/vms-volcanogenic-massive-sulphide-
deposits-ore-mineralization
Fig: Submarine volcanism

24. EVAPORATION PROCESS
◦ These deposits can form from the evaporation of lake and sea water.
◦ Evaporite is a name for water soluble minerals that originally precipitated from brine solutions and concentrated and
crystallized by solar evaporation from an aqueous solution.
◦ Excellent indicators of palaeo-climate. A hot climate is needed for major evaporate deposits to form.
◦ When evaporation occurs in bodies of saline water, concentration of the soluble salts occurs and when super saturation of
any salt is reached, the salt is precipitated.
◦ Deposition of minerals by evaporation depends on super saturation, which in turn depends upon other factors, chief of
which are:
a) temperature
b) pressure
c) depositional environment, and
d) seasonal & climatic changes

25. TYPES OF EVAPORITE DEPOSITS
1 Marine- Which can also be described as ocean deposits, the salts of the
oceanic waters are mainly the soluble products of weathering of the
rocks and of solution by subsurface waters.
2 Non-marine- Which are found in standing bodies of water such as
lakes . E.g. Mandi Salt Belts, Himachal Pradesh, Bikaner Evaporite
Basin.
3 Deposition from hot springs- Substances contained in hot-spring
waters build up deposits around their orifices. The chief substances
deposited in this manner are calcium carbonate, in the form of tufa,
travertine; silica in the form of siliceous sinter; iron oxide in the form of
ocher; and manganese dioxide in the form of wad.
4 Ground water evaporation- Groundwater contains salts similar to
those of the ocean and salt lakes but their concentration is low and the
proportion of individual salts may vary according to the character of the
soil, bedrock, topography and climate.
Fig: Salt deposit, Dead Sea, Israel
Fig: Salt Deposits, The Salinas Grandes Dry Salt lake, Argentina

26. METAMORPHISM AND CONTACT METAMORPHIC DEPOSIT
Metamorphism greatly alters the pre-existing mineral
deposits and form new ones.
Ore deposits formed due to metamorphism are of two
types-
1. Deposits formed due to metamorphism of pre-
existing ore deposit.
2. Deposits formed due to formation of new minerals
during metamorphism of certain rocks.
Contact metamorphic deposits are minerals formed at
or near the contact of a magma during a magmatic
invasion and are pneumatolytic in origin. The invaded
rocks become baked and hardened, their constituents are
recrystallized and recombined.
Limestone and dolomite are recrystallized to marble.
Other ore minerals consist of oxides of iron, native
minerals like graphite, gold and platinum.
https://study.com/academy/lesson/contract-metamorphism-vs-regional-
metamorphism-definition-differences.html

27. CONCLUSION
◦ There are numerous processes of formation of economic mineral deposits.
◦ The different processes of formation have their unique way of act upon the component they dealt with.
◦ Each and every ore deposits by their processes is given a defining name to it.
◦ The progress and development of a country greatly depends on the availability of economic mineral deposits.
◦ Hence a detail study about the processes of origin of these deposits is important so that other locations where
these minerals may occur can be located.

28. REFERENCES
◦ Economic Mineral Deposits by M.L.Jensen and A.M.Bateman
◦ Ore Deposits by Park and Mac Diarmid
◦ Economic Geology by Umeshwar Prasad
◦ https://www.Wikipedia.com/ore_genesis
◦ https://www.britannica.com/formation_of_ore_deposits
◦ https://www.google.com/ore_deposits