2. ORE DEPOSITS FORMED BY
and form new
The chief agencies
involved are heat,
acted upon are
mineral deposits are
formed from rocks
chiefly by the
crystallization and the
combination of rock
4. Role of Temperature and Pressure
processes occur to
between the chemical
potential of any
system and the
A particular chemical
reaction that cannot
occur in one
readily do so under
An increase in
pressure will cause a
reaction to move in a
direction in which the
total volume of the
5. An increase in temperature normally results in
eg:conversion of pyroxene to hornblende during
the metamorphism of diabase to amphibolite.
In short, metamorphic reactions result from the tendency of
mineral systems to adjust to their physicochemical environment
of high temperatures and pressures in contrast to the low
temperatures of weathering processes, both of which processes
generally occur in the presence of water.
7. Metamorphism of Earlier Deposits
When rocks are metamorphosed, enclosed mineral deposits may also be
Unlike rocks that undergo both textual and mineralogical changes, Ores
undergo less mineral re-combinations.
Textual changes, however, are pronounced. Schistose or gneissic textures
are induced, particularly with sectile minerals, and flow structure is not
Galena, for example, becomes gneissic. It may also be rendered so fine grained
that individual cleavage surfaces cannot be discerned with a hand lens.
8. The original textures and structures may
be so obscured that it is difficult to
determine to which class the originally
deposits belonged. Such deposits are
then classified as "metamorphosed".
The result is that ores may exhibit
streaked, banded, smeared
appearances with indistinct
boundaries between minerals of
It "flows" around hard minerals, such
as pyrite. Other minerals, such as
chalcopyrite, bornite, covellite, or
stibnite, behave similarly.
situations of intense
grains, atoms or
molecules of a rock
or mineral are packed
creating a new crystal
It is a physical
rock that undergoes
and clays can
11. Contact Metamorphism
Heat and chemical
reactions with hot fluids
from the magma can
change the composition
of the surrounding rock.
This process is called
It also occurs when hot
move through small
cracks in a large mass of
Ores, such as copper Cu
and zinc Zn, form by
13. Formation of Mineral Deposits by
Several kinds of
are formed as a
result of regional
materials are rock
Rarely, water or
carbon dioxide has
been added, but
not introduced as
they are in contact
rocks are wholly or
it is the rock
that has given rise
to the deposits.
14. The chief deposits
thus formed are
garnet, and possibly
15. a) Asbestos forms by the metamorphism (hydration) of ultrabasic igneous rocks – peridotites and dunites.
b)Graphite forms by regional metamorphism of organic matter, crystallization from igneous rocks, contact
metamorphism and hydrothermal solutions.
c)Talc, soapstone and pyrophyllite form by a mild hydrothermal metamorphism of magnesium minerals e.g. :
tremolite, actinolite, olivine, epidote and mica. Talc also occurs in regionally metamorphosed lime stones,
altered ultrabasic igneous rocks, and contact metamorphic zones.
d) Andalusite-kyanite-sillimanite – these minerals are high grade refractories. Kyanite is formed by the
dynamo thermal metamorphism of aluminous silicate minerals. Andalusite is formed by the pneumatolytic
action on aluminous silicates. Sillimanite results from high temperature metamorphism of aluminous
e) Garnet forms during the regional and contact metamorphism and is consequently found in schist's and
gneisses. It is also found as a constituent of igneous rocks.
f)Emery is a mixture of corundum and magnetite with hematite or spinel and is a product of contact
16. Bedded manganese deposits in parts
of Madhya Pradesh and Maharashtra,
primarily of sedimentary origin, have
been subsequently effected by
Braunite, a manganese silicate, is the
important ore mineral besides several
other oxide minerals.
18. Copper-Rich Types:
associated with very low grade to low grade metamorphism.
they form in terrains where mafic or ultramafic basement rocks
are overlain up section by organic-rich sedimentary rocks.
Richest zones are usually in close proximity to a fault or
other structure where fluids migrate.
White Pine, Michigan: Ore
= chalcocite + bornite +
chalcopyrite + minor
subaerial basalts overlain
22. Archean Iron Formation Types
veins in the
25. Quartz-Carbonate Types
Associated with greenstone belts in
Serious deformation of host rocks.
Ore formed in structural zones/shear
zones which are regional in scale.
The districts usually contain large
scale folding as well.
Associated with green schist facies
rocks, namely greenstone.
27. Lead-Silver-Rich Types
and chalcopyrite as
The gangue is
Mineral zoning is from
galena + sphalerite in
the lower portion of
the veins to galena +
siderite in the upper
portion of the veins.