Contact metasomatism forms new minerals through reactions between intrusive rocks and escaping gases from magma chambers. Important requirements include a magma source of ore ingredients and intrusion into reactive host rocks. Metals like Fe, Cu, Zn, and W can be deposited through this process. Hydrothermal deposits are formed when hot, mineral-laden waters circulate through fractures, leaching and redepositing metals. Sedimentary deposits can form through evaporation, biochemical processes, or mechanical concentration of minerals in placer deposits.

1. Contact Metasomatism
• Contact metasomatism is a process of formation of new mineral by
reaction between the contact rock and the escaping high temperature
gaseous emanation with other important materials from the magma
chamber.
• For the deposit of this type, the magma must contain the ingredients
of mineral deposit and must be intruded at depth at the contact of
reactive rocks.
• PROCESS:
1. Recrystallization and recombination of rock mineral in the contact
zones, e.g. limestone and dolomite convert to marble, shale to
hornfels and sandstone to quartzite.
2. The temperature for contact metasomatism may possibly ranging
from 400 to 800⁰ C, or even higher.
3. The composition, size, shape, form and depth of formation of the
intrusive body play important role in formation of contact
metasomatism.

3. • Important sources of ore deposit are Fe, Cu, Zn, W, and other
metals.
• Barytes occurs in limestone of vempalle formation (cuddaph),
Andhra pradesh.

4. Hydrothermal deposit
• Hydrothermal mineral deposits are those in which hot, mineral laden
water (hydrothermal solution) serves as a concentrating,
transporting, and depositing agent.
• The solutions are thought to arise in most cases from the action of
deeply circulating water heated by magma. Other sources of heating
that may be involved include energy released by radioactive decay
or by faulting of the Earth's crust.
• Conditions necessary for the formation of hydrothermal ore deposits
include:
a) Presence of hot water to dissolve and transport minerals
b) Presence of interconnected openings in the rock to allow the
solutions to move
c) Availability of sites for the deposits, and
d) Chemical reaction that will result in deposition

5. Hydrothermal
Deposits
Hot water and gases
circulate through
fractures in crust
Metal ions leached
from rock at depth are
concentrated and
redeposited
Gold, zinc, lead,
copper
N. Lindsley-Griffin, 1998
Woods Hole Oceanographic Institution
Hot water and sulfide
particles issuing from a
black smoker, East Pacific
Rise
Sulfide
minerals
deposited here
Mineral Deposits

6. Hydrothermal deposits in
ophiolites (on-land fragments of ocean
lithosphere)
Veins are deposited
along fractures in
basalts of oceanic crust
-
Divergent margins,
oceanic rift valleys
Ores are transported by
subduction and plate
movement, emplaced
on land by terrane
accretion in ophiolites -
Convergent margins,
active continental
margins
Mineral Deposits
Houghton Mifflin, Dolgoff, 1998; N. Lindsley-Griffin, 1999

7. Hydrothermal ore deposits
are forming today in the
Imperial Valley of California -
a graben formed by rifting
along the northern end of the
East Pacific Rise which runs
up Gulf of CA.
Metallic ions are leached from the
sediments under the graben by hot
fluids resulting from volcanism.
Hot brines deposit siliceous scale
containing 20% copper and 8%
silver on the insides of pipes in
drilled wells.
Fig. B16.1, p. 494
Mineral Deposits
N. Lindsley-Griffin, 1999

8. Hydrothermal deposits forming
today in the Red Sea:
Brines remain pooled in the deep graben because they are denser than
sea water.
This hydrothermal deposit is called a stratabound deposit, because the
minerals are precipitated as layers interbedded with sediments.
Mineral Deposits
N. Lindsley-Griffin, 1999
Fig. B16.2, p. 495

9. Metamorphic deposit
• Metamorphic processes profoundly alter pre-existing mineral deposits
and form new ones.
• The chief agencies involved are heat, pressure, time, and various
solutions.
• The materials acted upon are either earlier formed mineral deposits or
rocks.
• Valuable nonmetallic mineral deposits are formed from rocks chiefly by
the crystallization and the combination of rock making minerals.
• Several kinds of nonmetallic mineral deposits are formed as a result of
regional metamorphism.
• The source materials are rock constitutions that have undergone
recrystallization or re-combination, or both.
• Rarely, water or carbon dioxide has been added, but other new
constitutions are not introduced as they are in contact metasomatism
deposits.

11. • The enclosing rocks are wholly or in part metamorphosed; it is the rock
metamorphism that has given rise to the deposits.
• The chief deposits thus formed are asbestos, graphite, talc, soapstone,
andalusite-kyanite-sillimanite, dumortieritea, garnet, and possibly some
emery.

12. Sublimation
• Process by which solid material passes into gaseous state without
first becoming liquid.
• Sublimation is a process of mineral deposits associated with
volcanism, thermal springs and fumaroles where volatilised matter is
redeposited at lower temperature and pressure.
• Sulphur and Borax of Puga area, Ladakh are examples of such
deposit.
• They are associated with thermal springs and fumaroles.

13. Surfacial oxidation and supergene enrichment
• When ore deposits are exposed to the oxidation zone they are
weathered and altered with the country rocks.
• The surface waters oxidize many ore minerals and yield solvents that
dissolve other minerals.
• An orebody 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.
• The effects oxidation may, however, extend far below the one of
oxidation.
• As the cold, dilute, leaching solutions trickle downwards, they may lose
a part or all of their metallic content within the zone of oxidation to give
rise to oxidized ore deposits.
• The oxidized or near-surface part of an orebody is made colorful due to
the oxidation of sulfides to oxides and sulfates.

14. Contd………
• As the down trickling solutions penetrate
the water table, 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, unaffected part of the ore
body is called the hypogene zone.
• In some places the supergene zone is
absent and in rare cases the oxidized
zone may be shallow or lacking (as in
some glaciated areas undergoing rapid
erosion).
• Special conditions of time, climate,
physiographic development and
amenable ores are necessary for the
process of oxidation and supergene
enrichment to be effective.
• Such ores occur in most of the non-
glaciated land areas of the world.

15. Deposits in the zone of oxidation
When the oxidised zone is well developed and the secondary minerals
sufficiently concentrated, it is a highly profitable zone to mine as the
processing is much cheaper and easier and the metals more
concentrated. However, most oxidised zones have been mined because
they formed outcrops of easily identifiable gossans. The most common
minerals found in oxidised zones are:
• Copper: malachite, azurite, chrysocolla
• Gangue minerals: quartz (usually cryptocrystalline), baryte, calcite,
aragonite
• Iron: goethite, hematite
• Lead: anglesite, cerussite
• Manganese: pyrolusite, romanechite, rhodochrosite
• Nickel: gaspeite, garnierite
• Silver: native silver, chlorargyrite
• Zinc: smithsonite

16. Deposits in the zone of supergene enrichment
In the supergene zone metals are concntrated in a narrow band just
below the water table. This is the richest part of an ore deposit but in
many instances, is either only very thin or not developed at all. The
most common minerals found in supergene zones are:
• Copper: chalcocite, bornite
• Lead: supergene galena
• Nickel: violarite
• Silver: acanthite, native silver
• Zinc: supergene sphalerite, wurtzite

17. Sedimentary deposits form
by evaporation and
precipitation
Anhydrite, gypsum,
halite
Evaporite Deposits at Bonneville Salt Flats,
Utah N. Lindsley-Griffin, 1998
Mineral Deposits

18. Sedimentary deposits form by
biochemical reactions in
seawater
Banded iron formations were precipitated by biochemical
reactions in a low-oxygen atmosphere during the
Precambrian
N. Lindsley-Griffin, 1998
Mineral Deposits
Banded Iron Deposit, Lake Superior

19. Mechanical
Concentration
Placer deposits:
Heavy grains sorted
by currents
Deposited in rivers
or beaches
Previously weathered
from bedrock source
Gold, platinum,
diamonds, chromite,
Zirconium and Titanium
Olivine beach placers, South Point,
Hawaii
N. Lindsley-Griffin, 1998
Mineral Deposits

20. Placers are
deposited:
Behind rock bars
In rock holes
Below waterfalls
In point bars inside
meander loops
Downstream from a
tributary
Along beaches and
behind undulations
on the ocean floor.
Mineral Deposits
N. Lindsley-Griffin, 1999

21. Residual mineral deposits
form by chemical
weathering
Soluble minerals are leached - dissolved by rain water and
carried downward by infiltration, leaving behind less soluble
minerals.
Laterites are mined for iron and sometimes nickel.
Mineral Deposits
N. Lindsley-Griffin, 1999
Iron ore, Australia

22. Residual mineral
deposits
Bauxite is the main
source of aluminum
ore - found in
laterites formed in
tropical climates.
Mineral Deposits
N. Lindsley-Griffin, 1999
Bauxite (aluminum ore)
Weipa, Australia
Fig. 16.26, p. 499