1. ORE DEPOSITS THE ORE-BEARING
Kunwar Shruten Chauhan
UNIVERSITY OF RAJASTHAN
2. Ore Deposits and Plate Tectonics
Ore deposit - Rocks containing minerals of economic value in such amount that they can be profitably
ore genesis explain how the various types of mineral deposits form within the Earth's crust
Plate tectonics provides the distribution and origin of mineral deposits. The relationship of plate tectonics
and mineral deposits is significant on three counts:
Geological processes operating due to energy released at plate boundaries control the process of
Mineral deposits form in particular tectonic settings which are governed by plate tectonics.
Reconstruction of fragmented continents can provide a useful basis for exploration of new
I. DEPOSITS AT OCEANIC RIDGES (DIVERGENT PLATE MARGINS)
Hydrothermal activity at the ridges gives rise to a) Sulfide deposits and b) Metalliferous
sediments on the flanks of ridges. Important metallic deposits formed are Fe,Zn, Cu, Pb,Au and
Mn oxide deposits are important at some ridges Eg the TAG Hydrothermal field on the Atlantic
Ultramafic rocks in ophiolites containing asbestos,chromite and nickel ores. These are generally
accessible in Phanerozoic orogenic belts to which sites they have been transported due to plate
Podiform chromite deposits associated with serpentinized ultramafic rocks.
Cyprus Type massive sulfide deposits (Cu-Fe rich) are also associated with ophiolites and
represent hydrothermal deposits formed at ocean ridges.
II. DEPOSITS AT CONVERGENT PLATE MARGINS
Metallic deposits are commonly found at both continental and arc convergent plate margins. Along the
Circum-Pacific Belt major metallic deposits occur in western North and South America, Japan,
Philippines, New Zealand and Indonesia. More than half of the world's supply of copper comes from the
Porphyry Copper Deposits of this region. Important deposits associated with present and former
convergent margins are:
Base metals (Cu, Pb,Zn, Mo).
Precious metals (Pt, Au, Ag).
Other metals (Sn, W, Sb, Hg). (Red Bed uranium deposits are also associated with convergent
boundaries Eg SW United States).
Zoning of mineral deposits forming at convergent margins is apparent Eg in the Andes, going from west
to east,the various zones encountered are:
a) contact metasomatic Fe- deposits;
b) Cu-Ag and Ag veins;
c) Porphypy Cu-Mo deposits;
3. d) Pb-Zn-Ag veins and contact metasomatic deposits; and
e) Sn deposits.
Petroleum occurs in the back-arc basins in arc convergent margins
Potential geothermal fields also occur along convergent margins.
III. DEPOSITS AT COLLISION BOUNDARIES
Most of the deposits that occur here in the collision zones are variety of metallic deposits are abundant
Deposits generally related with oceanic ridges (ophiolites).
Those associated with convergent plate margins.
Mineral deposits associated with cratonic assemblages.
Deposits associated with continental rifts.
Deposits genetically related to collision zones are hydrocarbons which may accumulate in
foreland basins associated with such zones, Eg the Persian Gulf SW of the Zagros Suture in Iran.
IV. DEPOSITS IN CRATONIC RIFTSYSTEMS
Regional uplift and doming usually result when a continent comes to rest over a hotspot and huge
volumes of magma rise to the surface. Extensional failure of the lithopheric crust may occur with
continued doming, triggering the development of a triple junction - a three armed continental rift system.
Typically, one arm of the rift fails remaining a fissure in the crust known as an aulacogen, while the
remaining two arms open to form an oceanic basin. The prevalence of three armed rifts is revealed by
reassembling the continents surrounding the Atlantic Ocean to their positions before Pangea split up. In
most cases two of the arms were incorporated into the Atlantic, while the third remained as a blind rift
extending into the continent.
Rifting follows crustal doming in response to hot-spot activity in the mantle.
Granites intruded at this stage have associated Sn and fluorite deposits.
Evaporites accumulate in the rifts during the more advanced stages, with Pb-Zn-Ag deposits in
limestones forming during the early and middle stages of rifting. These are followed by oceanic
Aulacogens are characterized by the presence of fluorite, barite, carbonatites (with Nb, P,REE,
U, Th etc) and Sn-bearing granites.
Geothermal fields occur along the rifts due to the upwelling of the asthenosphere.
Carbonatites (unusual igneous rock rich in calcite and other carbonate minerals which are
considered to be mantle derived), kimberlites, and alkaline granites within or adjacent to rifts
provide a major source of metallic and other minerals.
V. DEPOSITS IN CRATONIC BASINS
Marginal and intracontinental cratonic basins provide a favourable setting for accumulation of organic
matter. During the opening of a cratonic rift, seawater moves into the basin and evaporation exceeds
inflow, giving rise to the formation of evaporites. The environment is characterized by restricted
4. circulation and hence organic matter is preserved leading to the accumulation of petroleum. With
continued rifting, circulation becomes unrestricted and deposition of evaporites and organic matter ceases.
High geothermal gradients beneath the opening rift and increase in pressure due to burial by sediments
facilitates the conversion of organic matter to petroleum. In the final stages of the opening of the basin,
the salt beds may begin to rise as salt domes forming traps for oil and gas. Oil and gas may also be
trapped in structural and stratigraphic traps as they move up due to increasing temperature and pressure,
Eg the Red Sea.
This speculation is lent support by the fact that around the Atlantic there is a close geographic and
geologic relationship between hydrocarbons and salt accumulations.
Ore bearing fluid
The four most important considerations in the formation of ore deposits are:
1. Source and character of ore-bearing fluids.
2. Source of the ore constituents and how they were obtained in solution.
3. Migration of ore-bearing fluids.
4. Manner of deposition.
For a closer examination the ore-bearing fluids are divided into four categories:
1. Magmas and magmatic fluids (silicate-dominated magmas or derived oxide, carbonate, or sulfide-rich
magmatic liquids )
2. Meteoric waters
3. Connate waters
4. Fluids associated with metamorphic processes.
5 H2O dominated hydrothermal fluidsfrommagmas
If the fluid is heated and liquid the aqueous fluids can be considered a hydrothermal solution
If the fluid is a gas, it is called pneumatolytic above the critical point, we are dealing with supercritical
MAGMATICWATER: Magmatic water or juvenile water is water that exists within, and in
equilibrium with, a magma or water-rich volatile fluids that are derived from a magma.
These are derived during differentiations and factional crystallization
Methods of concentration:
1) Fractional crystallization
2) Filter pressing
3) Magmatic Injections
4) Immiscibility (oxide and/or sulfide dominated magmas)
The metals deposited are related to the composition of the magma.
Mafic and ultramafic rocks: Cr, Ni, Pt,(PGE) and sometimes P
Silicic rocks: Sn, Zr, Th
5. I-type: Cu, Mo, Zn, Pb, Ag, Au
S-type: Sn, W, Be,U, Li
Ti and Fe are found in a range of compositions
a) Chalcophile elements: partition into a sulfide melt. Ni, Cu, Fe, Co, PGE
b) Siderophile elements partition into oxide melts (Cr, Fe, Ti, V, and P
c) Lithophile elements prefer the silicate melt
d) Compatible vs. incompatible Elements.
HYDROTHERMALFLUIDS Hot waters that usually have been derived from a magmatic process,or
warmed during magmatism.
These are the dregs of the crystallizing magma. (usually felsic) They can separate out at depth or
closer to the surface. The composition of these fluids is derived from:
1. The composition of the melt,
2. The P and T of separation,
3. The nature of other waters with which it interacts, and
4. Reaction with wallrock.
These fluids may contain
Pb, Zn, Ag, Au
LIL large ion Lithophile such as Li, Be,Rb
How do they precipitate metals?
a) The solubility of volatile and non-volatile solutes in supercritical water is proportional to
density. So if we go from high to low P and T, results in precipitation.
b) Cl complexes dissociate approximately as hydrothermal fluid density decreases,so alkali and
base metal complexes dissociate at lower P-T.
c) Boiling will cause precipitation: primary, when VP increased due to crystallization; secondary
when pressure release
Relative stability of Cl complexes:
High PT Low PT
Wall-rock alteration A reaction of hydrothermal fluids with enclosing rocks, causing changes in
mineralogy that are most marked adjacent to the vein and become less distinct further away.
There are two categories of wall rock alteration:
a. Hypogene - Hypogene alteration is caused by ascending hydrothermal fluids,
b. Supergene - By descending meteoric water reacting with previously mineralized
6. c. A third type of mechanism giving rise to the formation of wall rock alteration is the
metamorphism of sulfide deposits.
Types of wall rock alteration include
(1) Advanced argillic alteration - . This alteration is characterized by Dickite, Kaolinite
Pyrophyllite and quartz. Sericite is usually present and frequently alunite, pyrite, tourmaline, topaz
and zynyite. At high temperatures andalusite may be present.
(2) Sericitization - characterized by the assemblage quartz + sericite + pyrite.Sulfides present,in
addition to pyrite, include chalcopyrite, bomite and a variety of less common copper sulfides.
(3) Intermediate argillic alteration - The principal minerals are Kaolinite-and Montmorillonite group
minerals occurring mainly as alteration products of plagioclase.
(4) Propylitic alteration - This is a complex alteration generally characterized by chlorite,
epidote,albite and carbonate (calcite, dolomite or ankerite). Minor sericite, pyrite and magnetite may be
present, less commonly zeolites and montmorillonites’
(5) Chloritization - Chlorite may be present alone or with quartz or tourmaline in very
simple assemblages; however,other porpylitic minerals are usually present,and anhydrite may also be
(6) Dolomitization - addition of magnesium to limestone to form dolomite. Common in Mississippi
Valley type deposits.
(7) Potassic alteration - Secondary potash feldspar and/or biotite are the essential minerals of
this alteration. Clay minerals are absent but minor chlorite may be present.
(8) Silicification - characterized by quartz or chert. Can be added by solutions as is the case in many low
temperature deposits or the result of complete leaching of all cations plus aluminum.
(9) Feldspathisation - - kspar + albite,forms in the deep zones of some porphyry copper deposits.
(10) Tourmalinization - his is associated with medium to high temperature deposits, e.g., many tin and
some gold veins have a strong development of tourmaline in the wall rocks and often in the veins as
Oceanic Water. (Essentialto supergene processes.) Water from the atmosphere is meteoric water. It is
especially important in supergene processes. Temperature,and consequently solubility of minerals
increases as this water percolates down. Meteoric water contains the dominant crustal elements viz., Na,
Ca, Mg, SO4
etc,and it can get mixed with magmatic water.
SEAWATER - seawater as an ore-forming fluid are best described in the contexts of evaporates,
phosphorites, submarine exhalalites
Deposits related to seawaters are:
1) Alpine Peridotite chromite,
4) Mt Isa and Broken Hill???
5) Sedimentary iron deposits
6) Phosphate deposits
7. 8) Manganese modules
CONNATE WATERS: Water trapped in sediments at the time of their formation (fossil water). This
water often has a high mineral content and where formed in marine conditions can result in highly saline
groundwater. Contain Na,Cl, Mg, HCO3,Sr, Ba, and N compounds. They have little direct relationship
with ore-bearing fluids except when the strata containing them are undergoing metamorphism. When
activated, they become strong solvents of metals since they contain chlorine. They are thus one of the
sources of hydrothermal fluids. water trapped in sediments at the time they were deposited is known as
connate water connate waters are fossil waters observed in oil field exploration rich in sodium and
chloride, also considerable amounts of calcium, magnesium, and bi-carbonate, and many contain
strontium, barium and nitrogen compounds can also contain light hydrocarbons stable isotope ratios near
SMOW (Standard Mean Oceanic Water.) Mississippi Valley type deposits
FLUIDS ASSOCIATED WITH METAMORPHICPROCESSES: Connate and meteoric waters are
set in motion during metamorphism. They are chemically reactive due to heat and pressure,and are
therefore active ore carriers. They leach metals from surrounding rocks and from those through which
they move, and deposit them in areas of low temperature and pressure or reactive wallrock. Metamorphic
waters can also be derived from the breakdown of hydrous minerals (clay minerals contain as much as
14% water). These waters move down in advance of regional metamorphism (down the metamorphic
gradient) or the intruding magma.
Migration of ore-bearing fluids:
How various fluids move through rocks?. All fluids migrated at all levels in the earth’s crust.
Movement under the ground is considered as a very significant factor in ore genesis.
Concentration of oil, gas, emplacement of dikes,etc
Viscosity, density, permeability, structures, depth
1. Migration ofmagma
a. Magma is in general buoyant moved upward to areas of lowest pressure and temperature
b. Contained dissolved gases and water
c. Injected into overlying rocks, breaking rocks- Sills and dikes.
d. Mechanism of intrusion.
e. Fluids can move in rocks of exceedingly low permeability if sufficient time is allowed.
2. Porosity and Permeability
Porosity is the ratio of pore volume to total volume of a given rock or soil weather those pores
are interconnected or not, these interconnected pores help to migrate the ore bearing fluid.
Porosity may be primary, secondary and intersic.
8. Permeability describe the capacity or rock to transmit or permit fluid to flow. Permeability is
important required factor from movement of fluid. The permeability may be primary
secondary.They also classified as capillary, subcapillary, supercapillary.
3. Migration ofAqueous Fluids at Great Depths:
2) Ordinarily permeability and porosity both decrease with depth in the crust because the pressure of
overlying rocks tends to close any openings.
3) Nevertheless,large amounts ore-bearing solutions do move through tight rocks at depth. The
answer is time.
9. 4) Permeabilities of most near-surface rocks - those within 2-3 km of the surface - can transport
large amounts of fluids, enough to figure in significantly into ore deposit formation.
5) A fracture zone of 5milidarcy can transmit litres per year.
4. Migration ofAqueous Fluids at ShalowDepths:
1) Geologists have suggested that hydraulic pressures in ore-bearing fluids can keep fissures open,
allowing the fluids to circulate and permitting time for reaction and deposition.
2) The apparent impermeability of many rocks such as fresh granitic plutons indicates that
superimposed permeability due to faults and other secondary structures is more important than
3) Shallow drill hole release fluid under very high pressure.
5. Migration ofMetals in the Colloidal State:
A colloidal system consists of two phases:
1. The dispersed phase - is the diffused phase
2. The dispersion medium - in which the diffused phase is dispersed.
a) Colloidal particles range in size from those in true solutions to those in coarse suspension. The
limit of size are 10-3
cm (> solution < coarse suspension).
b) The colloidal material may be solid, liquid or gas and may be dispersed in one of these same
c) In the study of ore transport, we are concerned with solids suspended in liquids or a gaseous
d) A colloidal system consisting of solids dispersed in a liquid is called a Sol.
e) Colloidal particles have large surface areas per unit volume. Ions absorbed on the surfaces of
such particles control their behaviour.
f) If the particles absorb cations they become positively charged, if they absorb anions they become
g) These charges prevent the particles of the sol from coagulating or flocculating, but if an
electrolyte is added the particles neutralize and flocculate.
h) Most sulfides and organic sols are negative, whereas most oxide and hydroxide sols are
positive. There are some exceptions, eg. colloidal silica is negative.
i) Colloids are most stable in cool, dilute solutions and in the presence of a second (protective)
colloid. Eg. colloidal gold is stable below 150oC and coagulates between 150-250oC. In the
presence of colloidal silica this colloidal gold is stable upto 350o
j) It is difficult to explain the colloidal migration of metals in depth because the rocks at depth are
dense and relatively impermeable.
k) Some geologists suggest that ore fluids change from solutions at depth to colloidal sols in the near
l) Evidence of this is furnished by certain minerals which are found in forms that suggest
flocculation from a sol.
Deposition of ores
Deposition ofMagmatic segregation deposit
10. Magmatic segregation is a process by which one or more minerals become locally concentrated during the
cooling and crystallization of a magma. Rocks formed as a result of magmatic segregation are called
magmatic cumulates. Extreme segregation can sometimes produce monomineralic cumulates; example
occurs in the Bushveld Igneous Complex of South Africa, where cumulus layers of chromite (iron-
magnesium-chromium oxide, the only chromium ore mineral) are encased in cumulus layers of anorthite
Three oxide ore minerals form magmatic cumulates: chromite, magnetite, and ilmenite. The world’s
largest chromite deposits are all magmatic cumulates; the largest and richest of these is in the Bushveld
Complex of South Africa. Cumulus deposits of magnetite make poor iron ores, because cumulus
magnetites invariably contain elements such as titanium, manganese,and vanadium by atomic
substitution—although vanadiferous magnetites are important as a source of vanadium. Most of the
world’s production of this metal comes from cumulus magnetites in the Bushveld Complex.
Iron sulfide is the principal constituent of most immiscible magmas, and the metals deposit by iron sulfide
liquid are copper, nickel, and the platinum group. Immiscible sulfide drops can become segregated and
form immiscible magma layers in a magma chamber. when layers of sulfide magma cool and crystallize,
the result is a deposit of ore minerals of copper, nickel, and platinum-group metals in a gangue of an
iron sulfide mineral. ore deposits of the world formed in this way are the Merensky Reef of the Bushveld
Complex, producer of a major fraction of the world’s platinum-group metals; the Stillwater Complex,
Montana, host to platinum-group deposits similar to the Merensky Reef; and the Norilsk deposits of
Russia, containing large reserves of platinum-group metals.
Carbonatites are igneous rocks that consist largely of the carbonate minerals calcite and dolomite; they
sometimes also contain the rare-earth ore minerals bastnaesite, parisite, and monazite, the niobium ore
mineral pyrochlore, and (in the case of the carbonatite deposit at Palabora in South Africa) copper sulfide
ore minerals. Most carbonatites occur close to intrusions of alkaline igneous rocks (those rich in
potassium or sodium relative to their silica contents) or to the ultramafic igneous rocks (rocks with silica
contents below approximately 50 percent by weight) known as kimberlites and lamproites. These
11. associations suggest a common derivation, but details of the way that carbonatite magmas might
concentrate geochemically scarce metals remain conjectural.
Deposition through hydrothermal fluid
These processes are the physico-chemical phenomena and reactions caused by movement of hydrothermal
waters within the crust, often as a consequence of magmatic intrusion or tectonic upheavals. The
foundations of hydrothermal processes are the source-transport-trap mechanism.
Most metals of economic importance are carried as trace elements within rock-forming minerals formed
by hydrothermal processes.
This happens because of:
a) incompatibility of the metal with its host mineral, for example zinc in calcite, which favours aqueous
fluids in contact with the host mineral during diagenesis.
b) solubility of the host mineral within nascent hydrothermal solutions in the source rocks, for example
mineral salts (halite), carbonates (cerussite),phosphates (monazite and thorianite) and sulfates (barite)
elevated temperatures causing decomposition reactions of minerals.
Sulfide deposition within the trap zone occurs when metal-carrying sulfate, sulfide or other complexes
become chemically unstable due to one or more of the following processes;
12. a) falling temperature,which renders the complex unstable or metal insoluble
b) loss of pressure, which has the same effect
c) reaction with chemically reactive wall rocks, usually of reduced oxidation state,such as iron
bearing rocks, mafic or ultramafic rocks or carbonate rocks
d) degassing of the hydrothermal fluid into a gas and water system,or boiling, which alters the metal
carrying capacity of the solution and even destroys metal-carrying chemical complexes.
Metal can also become precipitated when temperature and pressure or oxidation state favour different
ionic complexes in the water,for instance the change from sulfide to sulfate, oxygen fugacity, exchange
of metals between sulfide and chloride complexes, et cetera.
Ore deposits formed by lateral secretion are formed by metamorphic reactions during shearing, which
liberate mineral constituents such as quartz, sulfides, gold, carbonates and oxides from deforming rocks
and focus these constituents into zones of reduced pressure or dilation such as faults. This may occur
without much hydrothermal fluid flow, and this is typical of podiform chromite deposits.