Download Metallic Mineral Deposits (page 587)

Metallic Mineral Deposits (page 587)
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Mineral resources
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Ore – refers to useful metallic minerals that can be mined at a profit and, in
common usage, to some nonmetallic minerals such as fluorite and sulphur
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To be considered of value, an element must be concentrated above the level
of its average crustal abundance
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Most nonmetallic minerals are generally not called ores, but rather they are
called industrial minerals
Magmatic Deposits
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Some of the most important accumulations of metals are associated with
magma that forms igneous rocks
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Certain metals are enriched in certain magmas and further concentrated
during cooling of the magma
Magmatic Deposits: Gravitational Settling
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heavy minerals that crystallize early, settle and concentrate on the
bottom of the magma chamber
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Separation and non-mixing of liquid phases of a magma (e.g., Sudbury,
Ontario and Voisey’s Bay, Newfoundland and Labrador)
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Nickel deposits of Sudbury and Voisey’s Bay have similar origins, but
different triggering mechanisms
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Hydrothermal Deposits: one of the most important ore deposits. These
deposits are generated from hydrothermal (hot water) solutions. These
solutions of hot water and salts (brines) dissolve small amounts of metallic
elements. These brines are found around the magmas and they dissolve
certain minerals (gold, silver, copper, lead, zinc and mercury) and form
deposits in two ways.
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Hydrothermal Deposits
• Hydrothermal Deposits associated with Igneous Activity
– Among the best known and important ore deposits,
generated from hot-water solutions
– Majority originate from hot, metal-rich fluids that are
remnants of late-stage magmatic processes
– Move along fractures, cool, and precipitate the
metallic ions to produce vein deposits
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Vein Deposits: hydrothermal fluid generated by cooling magmas rise to
the surface through fractures, faults and other rock features. As these
fluids pass through there fractures, they cool and solidify. These fluids are
often silica (quartz) and calcite rich. When they solidify, they form veins
rich in Au, Ag, Cu, Zn, Pb and other metallic elements. These deposits are
referred to as vein or load deposits. See diagram on page 589.
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Hydrothermal Deposits
• Hydrothermal Deposits associated with Igneous Activity
– Can occur as disseminated deposits, which are
distributed throughout the rock body, rather than
concentrated in veins; called porphyry deposits = (low
grade; large volume)
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Disseminated Deposits: these deposits are also formed through magmasourced hydrothermal fluids, however rather than having concentration of
metals in veins, the metals are disseminated (spread out) through the
rock. Most of world`s copper and molybdenum and lesser amounts of
gold and silver are mined from disseminated deposits. These deposits are
often referred to as a porphyry deposits because they are associated with
porphyritic rocks. See page 589.
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Placer Deposits: minerals that deposited in streams, primarily gold. Think of
Dawson City Yukon, gold panning.
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Sedimentary Deposits
• Placer deposits – formed when heavy
mechanically concentrated by currents
• Examples include
– Gold, Platinum, Diamonds
metals
Mineral
Chalcopyrite
Sphalerite
Phyrite
Galena
Pentlandite
Gypsum
Halite
Bauxite
Borite
Copper ore
Zinc ore
Fool`s gold
Lead ore
Nickel ore
Evaporite
Evaporite
Aluminum ore
Drilling mud
CuFeS
ZnS
FeS
PbS
NiS
Gyproc
Rock salt
Natural Mineral: gold, silver, copper occur naturally in nature.
How do we find and then extract these mineral deposits?
Some of the exploration techniques involved are: Seismic records, remote sensing, prospecting,
observing drill core, cross sections, geologic mapping, magnetic surveys, gravity surveys,
geochemistry.
Once a deposit has been identified, there are two methods of extracting the minerals from the
ground.
Open Pit mining: Mineral extraction from the surface. The best example in Newfoundland is the
Iron ore mines of Lab City.
are
Underground Mining: Ore is extracted by a series of tunnels in the ground accessing a particular
deposit. A very local example would be the former Bell Island Iron ore mines.
Once these deposits have been accessed, the minerals must be separated from the host rock. Some
methods used are: Flotation, Gravity separation, Heap leaching, Pyromet, and Hydromet processes.
Floatation involves mixing grounded ore with water, oil, and
chemicals. The grounded ore becomes suspended in the water and
is subjected to bubbles of air due to the presence of chemicals. This
causes the minerals and/or elements of value to fl oat to the top where
they can be scraped off.
Gravity separation involves feeding grounded ore into a pulsating
body of water, which serves to settle out the heavy material while
floating away the light material. If the heavy material is what one
wishes to retain, then it is taken from the bottom. If the light material
is what one wishes to retain, then it is taken from the top.
Heap leaching commences by placing the grounded ore as a layer
onto impermeable material. The “heap” of material is irrigated with a
liquid, which percolates down through dissolving away the valuable
minerals and/or elements.
Pyromet involves using heat to separate the minerals and/or elements
of value from the ore. Heating separates the materials by density and
the desired minerals and/or elements can be removed.
Hydromet is a chemical method that involves oxidation and acid
leaching to separate out the desired minerals and/or elements.
Petroleum: A group of hydrocarbons that includes oil and gas.
Where does it come from? It comes from organic matter contained in the tissue of living things.
The vast majority of petroleum comes from the remains of simple, aquatic organisms (plankton algae
),which includes most of the plankton in the seas and large bodies of water.
Most of this algae is eaten by scavenging organisms and aerobic (oxygen rich) organisms.
This organic matter is said to have a low preservation potential in this type of environment. However in
quiet water environments in lakes and seas where conditions are low in oxygen, large accumulations of
plankton are “built up”.
Because it is a still, quiet geological environment, the rock type formed is organic rich, fine grained
sediments, such as shale and or fine grained carbonate rocks. These rocks are the principal sources of
petroleum and are thus called Source Rocks.
Different processes and temperature changes result in this organic matter forming gases and or liquid
petroleum.
As the hydrocarbon, rich, organic matter of source rock is subjected to higher temperatures ( through
burial ) , the matter is transformed to a waxy substance called Kerogen, (kerogen is a mixture of organic
matter in sediments from which petroleum is released). Then through a process called Cracking, the
carbon-carbon bonds within the kerogen break forming oil and gas, (this usually occurs at temperatures
above 50 degrees C.
The three phases in the evolution of organic matter to petroleum.
Include:
(i) diagenesis
(ii) catagenesis
(iii) metagenesis
Diagenesis: During diagenesis there is shallow burial of organic
matter at near normal temperature and pressure as well as some
decay. Methane, carbon dioxide, and water are released leaving
behind the complex hydrocarbon called kerogen.
Catagenesis: Deeper burial results in increased temperature and
pressure. Petroleum is released from the kerogen – first oil is released
and second gas is released.
Metagenesis: The metagenesis phase involves even higher
temperature and pressure verging on metamorphism. The only
hydrocarbon that is released during this phase is methane. At this
point the petroleum has matured enough to migrate to traps.
So to form a petroleum deposit we need a petroleum trap. A Petroleum Trapis an environment that
enables economically significant amounts of oil and gas to accumulate underground.
To have a petroleum trap there must be a reservoir rock and a cap rock.
A Reservoir Rock is a porous, permeable, rock that contains oil and or gas, such as sandstone.
A reservoir rock requires high porosity
and high permeability since it is the rock which petroleum moves
through and is stored in. Note that porosity is the volume of pore
spaces or holes between sediment grains and that permeability is the
interconnectiveness of the pores, thereby allowing the movement of
the petroleum. The porosity of a material is influenced by particle
shape and size and the degree of sediment sorting. Large, rounded,
well-sorted particles offer higher porosity, particularly if the amount
of cement between them is limited. Usually, the higher the porosity
and the larger the pore spaces, the higher the permeability. Examples
of reservoir rocks are sandstone, dolomite, and conglomerate since
they have both high porosity and permeability.
A Cap Rock is a rock layer overlying the reservoir rock to halt the movement of oil and gas to the
surface, such as shale.
A cap rock is an impermeable rock that
serves to trap petroleum from either escaping to the surface or
spreading throughout the rock as opposed to being confi ned. Students
should realize that petroleum exists within reservoir rock between
sediment as opposed to being confi ned as a whole volume of liquid
petroleum. “There are no ponds of petroleum in the ground”.
There are two main types of petroleum traps.
1) Structural Traps: Petroleum traps that are due to structural deformation of the rock layer that
contains the hydrocarbons.
There are three types: a) anticline
b) fault trap
c) salt dome trap
2) Stratigraphic Traps: petroleum traps that are due to lateral or vertical changes in
type that reflect patterns of sediment deposition.
There are three types: a) pinch-out trap
b) An unconformity trap c) Reef trap
Common oil traps – anticline, fault, salt dome, and stratigraphic
the rock
The physical property density is used for
the purpose of distributing petroleum in a reservoir. Water is denser
than oil and oil is denser than gas. Therefore, when drilling into a
petroleum trap, gas is encountered first followed by oil and then
water. Note that some reservoirs may have all three components,
whereas some reservoirs may only have two components or one
component.
There are two main ways oil is obtained from Earth.
1) Drilling eg.Hibernia
2) Surface extraction eg.open pit mining. The tar sands of Alberta.
The Alberta oil sands, also called tar sands, are an example of
petroleum being extracted directly from the surface (open-pit
mining). Because the petroleum migrated towards the surface and
volatiles (e.g. water) were lost to the atmosphere, the petroleum
experienced increasing viscosity (i.e. thickness), thereby preventing
it from spreading out and/or dissipating. The reservoir consisted
of loose (unconsolidated) sediment that therefore, exhibited high
porosity and permeability.
In some instances, steam is injected directly into the tar sands to
mobilize the hydrocarbons, which are then recovered from pumps
much like conventional crude oil.
The refining process involves three methods of refining petroleum. Include:
(i) distillation
(ii) cracking
(iii) reforming
Once petroleum is extracted from the sub-surface and is de-salted
and de-watered, it enters a distillation column. Heat is added to the
column to separate the petroleum into fractions based on boiling
points (cracking). The bottom fraction is removed to another column
for further separation. An image of a distillation column is shown below. Note
some “fractions” in the image, which could include: asphalt; paraffin
wax; lubricants; jet fuel; diesel; kerosene, and gasoline from the
graphic.
Source: http://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Crude_Oil_Distillation.
png/225px-Crude_Oil_Distillation.png
Reforming involves heat, pressure, and the use of catalysts (speed up
reaction rates) to reform different hydrocarbon compounds.
What are the environmental concerns of fossil fuels?
How can we satisfy the human appetite for fossil fuels and its ever increasing effects on the
environment?