Origin of Mineral Deposits

choppedspleenMécanique

21 févr. 2014 (il y a 3 années et 3 mois)

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Origin of Mineral Deposits




A
mineral deposit

is a volume of rock enriched in one or more
mineral
s.

In this sense a

mineral refers to a useful material, a
definition that is different from the way we defined a

mineral
earlier in this unit
.

Reference:

Pages 585
-

596

Chapter 21




Mineral deposits can be classified on the basis of the mechanism
responsible for concentrating

the valuable substance.

Examples
Include:

1)
Hydrothermal Mineral Deposits

2)
Magmatic Mineral Deposits

3)
Sedimentary Mineral Deposits

4)
Placer Mineral Deposits

5)
Residual Mineral Deposits

Origin of Mineral Deposits




Hydrothermal deposits are produced when groundwater circulates
down to depths

and heats up
,
either by coming near a hot igneous
body or by circulating to great depth
s

which naturally heat the
water because of
the geothermal gradient.

(30
EC

per km depth)

1)

Hydrothermal Mineral Deposits




Such hot water can dissolve valuable
minerals as it passes

through
a

large volume of rock.




As the hot water moves into cooler areas of the crust, the dissolved

substances are precipitated from the hot water solution.




Minerals are
concentrated by hot fluids

flowing through fractures
and pore spaces in rocks.

Origin of Mineral Deposits

1)

Hydrothermal Mineral Deposits




If the cooling takes place rapidly

in open fractures or upon
reaching cool surface water
s
, then

precipitation will take place
over a limited area, resulting in a

higher
concentration of
minerals

than was originally present in the rocks
.

Cooler water
moving toward
surface

Hot water
moving at
depths

Ore minerals
depositing in
veins

Examples:

1)
Massive sulfide deposits

2)
Vein deposits

3)
Stratabound mineral deposits

Origin of Mineral Deposits




Magmatic process such as partial melting,
fractional
crystallization
,
and

crystal settling in a

magma chamber can
concentrate ore minerals containing valuable
metals

by taking

elements and

concentrating them in minerals that separate from
the magma.

2)

Magmatic

Mineral Deposits




Minerals

are concentrated within a body of igneous rock

by
magmatic processes like

crystal settling.




As minerals
crystallize from a
magma body, heavy
minerals may

sink
to the bottom of the
magma chamber.

Magma
Chamber

Crystal
Settling

Origin of Mineral Deposits




These mineral
deposit
s

form as a result of
chemical sedimentation,
where minerals are precipitated directly out of water.

3)

Sedimentary

Mineral Deposits




Minerals

are concentrated by chemical precipitation

from
lake or sea water.

Examples Include:

Evaporite Deposits

-

Evaporation of lake water or sea
water results in the loss of

water and thus concentrates
dissolved
minerals

in the remaining water.


When the

water
becomes saturated
with

dissolved
minerals,
they precipitate
from the

water.

Deposits of
halite

(table salt)

and

gypsum

(used
in plaster and wall board),

result from this process
.

1)

Origin of Mineral Deposits




These mineral
deposit
s

form as a result of
chemical sedimentation,
where minerals are precipitated directly out of water.

3)

Sedimentary

Mineral Deposits




Minerals

are concentrated by chemical precipitation

from
lake or sea water.

Examples Include:

Iron Formations

-

These deposits are of iron rich minerals
that were deposited in
marine environments

during the

Proterozoic
.


These formed as a result of iron
-
rich waters
reacting with oxygen released by algae in the early stages of the
evolution of life. T
he composition of sea water must have been
drastically different than it

is today.

2)

Origin of Mineral Deposits




When the velocity of the water slows, minerals with a higher

density are deposited.

Heavy minerals like gold, diamond, and
magnetite will be concentrated in areas where water current
velocity is low.

The lighter minerals (quartz) are carried away.

4)

Placer

Mineral Deposits




Minerals

are concentrated by flowing surface waters
depositing high density minerals
either

in streams or along
coastlines.




G
old originally form
ed

in hydrothermal veins,
is
eroded out of the
veins and carried in streams where it was deposited in placer
deposits.

The California gold rush in 1849

began when someone
discovered rich placer deposits of gold in streams
.

Origin of Mineral Deposits




Placer deposits

occur in any area

where current velocity is low,
such as
;

4)

Placer

Mineral Deposits

1)
between ripple marks

Placer

Deposit

Stream Direction

3)
on the inside of
.

meandering streams

Placer

Deposit

2)
behind

rock

bars

Stream Direction

Placer

Deposit

4)
in holes on the bottom
.

of a stream

Stream Direction

Placer

Deposit

Origin of Mineral Deposits




Two common mineral deposits formed in this way are



iron
-
rich
Limonite

and aluminium
-
rich
Bauxite
.



Bauxite is the world’s
primary
source of aluminium
.

5)

Residual

Mineral Deposits




Minerals

are concentrated by chemical weathering

processes.




These deposits often form

as a result of chemical weathering
in
warm tropical climates that receive high

temperatures and high

amounts of rainfall

which produces
highly leached soils rich in
both iron and aluminium.

Chemical weathering tends to remove
the soluble materials, leaving the less soluble residues.


Origin of Mineral Deposits

5)

Residual

Mineral Deposits




In addition, an existing mineral deposit can be turned in to a more
highly concentrated

mineral deposit by weathering in a process
called
secondary

enrichment
.

Sample Problem

Answer:

Hydrothermal

-

hot solutions react with rocks in which they pass and as
a result the hot solutions becomes concentrated with metals in solution.
When the hot solution enters a cooler environment, the metals
precipitate from the solution and form metallic mineral deposits. Ex.
gold in quartz.

Depositional

-

form by a process of sedimentation. Mineral deposits
form as a result of chemical precipitation, evaporation, and density
deposits in water environments. Ex. gypsum and halite deposits or gold
accumulating in water environments as placer deposits.


Distinguish between hydrothermal and depositional methods of
the formation of economic mineral deposits.