Transcript Metal Deposits - Geology Rocks

Metal Deposits
The specification states that you should be able to:
 a) Explain the low crustal abundances of
metallic minerals; show an understanding of
concentration factors to produce economic
deposits; calculate concentration factors.
 b) Describe the concentration of magnetite by
gravity settling in igneous intrusions.
 c) Explain the formation of porphyry copper
deposits of chalcopyrite and the secondary
enrichment of copper deposits.
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Metal Deposits 2
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d) Explain the hydrothermal processes
associated with igneous intrusions,
sedimentary basins and ocean ridges forming
veins of galena, sphalerite and cassiterite.
 e) Explain how residual deposits of bauxite
are formed as the insoluble product of
chemical weathering.
 f) Explain the formation of placer deposits of
cassiterite and gold in rivers and beaches
and the characteristics of these minerals,
which make them suitable.
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If a rock or mineral deposit
can be mined economically
for a particular metal element
then it is called an ore body.
 Within an ore body there are
ore minerals and gangue
minerals (waste).
 The grade of an ore body is
related to the total % of the
metal element it contains.
 To be economic a copper ore
body needs 5- 10% Cu.
 Sometimes if the rock is
abundant and easily mined
the grade may be as little as 1
- 0.5%.
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Most metals have low average
crustal abundances and in
order for them to be worth
mining they must be
concentrated in some way.
 Look at the table on page
272 of McLeish to see:
average crustal abundances
cut off grades
concentration factors.
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The elements need to be
concentrated and this is done
either within the Earth or at the
Earth surface.
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MAGMATIC
SEGREGATION:
Examples include ores of
chrome and magnetite.
These minerals may form
first during crystallisation of
a basic magma and due to
their high density they may
sink to the base of the
intrusion and be
concentrated in layers.
The Bushveld complex in
South Africa is a good
example.
HYDROTHERMAL DEPOSITS:
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At the end of crystallisation of
acid magmas some elements
that do not easily fit into rock
forming minerals may be left in
a water rich volatile fluid.
These are called incompatible
elements.
These may mix with water rich
fluids and may escape in a
hydrothermal fluid.
These hot water rich fluids
(about 600C) move into
fractures or through pore
spaces.
HYDROTHERMAL DEPOSITS 2
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Metals dissolved in these fluids are precipitated in the
fractures or pore spaces.
 These fluids have a number of origins:
 1) From the final water rich fluids from an acid magma.
 2) Burial of sediments will mean that any trapped water
(connate water) will heat up and may flow through the
rocks I convection currents scavenging out any metals
within the sediments and concentrating the metal in the
hydrothermal fluids.
 3) Seawater at M. O. R.’s can circulate through the ocean
crust in convection currents and scavenge metals from the
crust. These fluids are then emitted from “black smokers”
precipitating metal sulphides into the oceans as sediment
and nodules.
HYDROTHERMAL DEPOSITS 3
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Hydrothermal deposits
are most common in
veins (filling joints or
faults) but if they are
precipitated in the pore
spaces they are called
Porphyry Copper Type
deposits.
 The order that the ores
are precipitated is in
order of cooling T and
so will vary along a
vein away from the
source.
Concentration at the Surface
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PLACER DEPOSITS
On weathering the rocks are
broken up and resistant ores can
be transported in rivers as solids.
The density of the ores effects the
distance they can be transported
and as soon as the energy drops
the dense ore will be dumped
amongst river sands and gravels.
The ore will concentrate where a
river moves from the mountains
onto the plane or on the inside of
meanders.
Cassiterite and gold can be
concentrated in this way.
They are dense and inert.
RESIDUAL DEPOSITS
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Chemical weathering acting on
rocks such as basalt can
weather them deeply under the
right conditions (humid and
warm).
The weathering will remove
material in solution but will
leave a gradually accumulating
layer of insoluble residue that
may be metal rich.
The best example is bauxite (Al
Ore) in places such as Jamaica
and N. Australia, which forms
from the weathering of basalt
and is in effect a very deep
fossil soil.
This is very easily mined.
SECONDARY ENRICHMENT (OF HYDROTHERMAL VEINS):
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This process leads to a very high
metal concentration.
 This usually happens close to the
water table as water plays an
important role.
 Above the water table oxidation
of the ore takes place which
changes insoluble sulphides into
soluble sulphates.
 These sulphates are carried
downwards by percolating water
down to the groundwater.
 Below the water table the
conditions are reducing and the
sulphates are reprecipitated as
sulphides.
SECONDARY ENRICHMENT (OF HYDROTHERMAL VEINS) 2
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If we take chalcopyrite as an
example then above the water table
this happens:
Chalcopyrite + O2 + H2O
Cu/Fe sulphide
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Chalcocite + Iron Hydroxide
Cu sulphate
Therefore the soluble compounds
are leached from the upper zones
and re-deposited lower.
The upper zone is reduced in Cu
although close to the water table
where oxidising conditions are less
effective CuCO3 can be deposited =
Malachite and Azurite.
SECONDARY ENRICHMENT (OF HYDROTHERMAL VEINS) 3
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Most of the Cu is carried
into the saturated zone
where the conditions are
reducing and it is
precipitated as copper
sulphide : chalcocite.
 Frequently a brown solid
residual deposit called
gossan forms which is
usually Fe sulphide left
behind after the Cu is
washed out and is a good
indicator of metal ores
below.