Mineral deposits

Origin of Mineral Deposits
¾ A mineral deposit is a volume of rock enriched in one or more
minerals. 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.

¾ 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
1) Hydrothermal Mineral Deposits
¾ Minerals are concentrated by hot fluids flowing through fractures
and pore spaces in rocks.

¾ 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 depths which naturally heat the
water because of the geothermal gradient. (300C per km depth)

¾ Such hot water can dissolve valuable minerals as it passes through
a large volume of rock.

¾ As the hot water moves into cooler areas of the crust, the dissolved

substances are precipitated from the hot water solution.



Origin of Mineral Deposits
1) Hydrothermal Mineral Deposits
¾ If the cooling takes place rapidly in open fractures or upon
reaching cool surface waters, then precipitation will take place
over a limited area, resulting in a higher concentration of minerals
than was originally present in the rocks.

Examples:

Ore minerals
depositing in
veins

Cooler water
moving toward
surface

1) Massive sulfide deposits
2) Vein deposits
3) Stratabound mineral deposits

Hot water
moving at
depths

Origin of Mineral Deposits
2) Magmatic Mineral Deposits
¾ Minerals are concentrated within a body of igneous rock by
magmatic processes like crystal settling.

¾ 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.

¾

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
3) Sedimentary Mineral Deposits

¾ Minerals are concentrated by chemical precipitation from
lake or sea water.

¾ These mineral deposits form as a result of chemical sedimentation,
where minerals are precipitated directly out of water.

Examples Include:
1) 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.


Origin of Mineral Deposits
3) Sedimentary Mineral Deposits
¾ Minerals are concentrated by chemical precipitation from
lake or sea water.

¾ These mineral deposits form as a result of chemical sedimentation,
where minerals are precipitated directly out of water.

Examples Include:
2) 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. The composition of sea water must have been
drastically different than it is today.

Origin of Mineral Deposits
4) Placer Mineral Deposits
¾ Minerals are concentrated by flowing surface waters
depositing high density minerals either in streams or along
coastlines.

¾ 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.

¾ Gold originally formed 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
4) Placer Mineral Deposits
¾ Placer deposits occur in any area where current velocity is low,
such as;

1) between ripple marks
Placer
Deposit

Stream Direction

2) behind rock bars
Stream Direction

Placer
Deposit

3) on the inside of

4) in holes on the bottom

. meandering streams

. of a stream

Placer
Deposit

Stream Direction

Placer
Deposit


Origin of Mineral Deposits
5) Residual Mineral Deposits
¾ Minerals are concentrated by chemical weathering
processes.

¾ 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.

¾ 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.


Origin of Mineral Deposits
5) Residual Mineral Deposits
¾ 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.