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OCR A LEVEL GEOLOGY > Economic and Engineering Geology > Flashcards

Economic and Engineering Geology Flashcards

1
Q

What is porosity?

A

Amount of pore spaces in a rock shown as a percentage

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2
Q

What is the equation for porosity?

A

Total volume of pore space ÷ total volume of rock/sediment x100

Or

% Mass change = final madd - dry mass ÷ dry mass x 100

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3
Q

Why is it difficult to achieve porosity above 26%?

A

Every grain is unequal in size

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4
Q

What is permeability?

A

Is the ability of a rock/sediment to transmit fluids

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5
Q

What is the equation for permeability?

A

Permeability = distance fluid travels ÷ time

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6
Q

What are some factors affecting permeability?

A

-Porosity
-Connectivity of pores
-Sorting
-Diagenesis
-Grain shape
-Packing
-Grain size
-Secondary permeability fractures (faults)

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7
Q

What are aquifers?

A

Very permeable rocks that can store water underground, such as chalk and sandstone.

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8
Q

What are aquicludes?

A

Very impermeable rocks that can’t store water such as clay and shale.

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9
Q

What is an aquitard?

A

A mix between and aquifer and an aquiclude.

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10
Q

What is connate water?

A

Trapped water in the pores of a rock as the rock formed. Includes water trapped in the original sediment and water released during diagnesis.

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11
Q

What is capillary pressure?

A

The pressure between two immediate fluids in narrow pore spaces resulting from interactions of forces between the fluids and solid grains.

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12
Q

What’s the different between a saturated and unsaturated material?

A

Saturated - All pore spaces are occupied by water
Unsaturated - Not all pore spaces occupied by water (is moving through)

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13
Q

What separates the saturated ans unsaturated zone underground?

A

The water table (the upper surface of the saturated zone)

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14
Q

What is the head relating to the water table

A

This is the height difference between the top of the water table and the point of escape

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15
Q

What is the equation for the hydraulic gradient?

A

Head ÷ horizontal distance between the two points

Usually in a ratio

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16
Q

How are cones of depressions made?

A

When water is abstracted from a well, a hydraulic gradient is created as the rate of eater flowing back into the well from surrounding saturated rocks will be greater at the top of the head. This creates a dome of depression as the rock towards the top is dried faster than below.

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17
Q

Name 4 case studies for aquifers and aquicludes.

A

London Basin - Artesian basins
Kessingland - Perched aquifer
The Bahamas - Aquifer
Horsey Mere - Aquifer
Sahara desert - Aquifer
The Great Artesian Basin

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18
Q

Explain the London Basin case study (Artesian basin)

A

London is at the bottom of a basin, with a Chalk aquifer dipping down from the North and then dipping upwards to the South again. This allows for an aversion fountain to be made due to the water coming from a higher place, so more pressure. London is on a clay aquiclude, so will need to drill to access water.

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19
Q

Explain what happens at the Kessingland perched aquifer

A

Water was perched on a clay lens, which, when dug through, leaked back into the surrounding rocks, so we’ll disappeared.

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20
Q

Explain the conditions of the Bahamas ans what happened with the aquifers there.

A

Conditions
-Low lying
-Sub-Topical
-Low rainfall
-High evapotranspiration

Aquifer
The water at the top of the lane, above sea level in the rocks, is fresh water from rainfall. Under the rocks, below the sea level, the water in the rocks is sea water. So when a borehole is drilled, they will mix to form brackish water

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21
Q

Explain the conditions at Horsey mere and what happens at the aquifer?

A

As Horsey mere is located behind dunes that separate it from the North Sea, salt water percolate through the sand and into the peat (incursion).
Fresh water from rain percolate through the sand and mixes with the salt water under the water table, creating brackish water.

It will change in salinity depending on the amount of precipitation.

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22
Q

Explain the conditions in the Sahara Desert and what happens at the aquifer?

A

-Arid
-Hot
- Formerly wetter
- Fossil groundwater

The exposed sandstone in the mountains where rain water percolate through. This then moves through the aquifer underground and comes back to the surface where sandstone is exposed.
This creates oasis’s

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23
Q

Explain the conditions of the Great Artesian Basin in Eastern Australia and explain what happens there?

A

European farmers used this for water supply for irrigation and livestock.
In recent years, attempts were made to stop bore holes from freely flowing
Additionally, 1.2km deep creates hot wa5er for geothermal power heated to 98°c in Birdsville, providing 25% of the electricity.

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24
Q

What is a confined aquifer?

A

Aquifer is covered by an aquiclude

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25
Q

What is an unconfirmed aquifer?

A

Aquifer that is exposed to the surface at some point

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26
Q

How does the storage of water differ between the North of England and the South?

A

South- groundwater (aquifers of Chalk)
North - surface storage (resinous in deep impermeable valleys)

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27
Q

Why is ths south east of England in a water defecit?

A

Dry, low rainfall and high population whereas north is in a surplus due to less population and high rainfall.

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28
Q

What are 3 advantages ans disadvantages of drinking groundwater?

A

Advantages
1) Naturally filtered - will be clear
2) Long history of “Health Benefits”
3) Big profits for water bottling companies
Disadvantages
1) Bottling can allow bacteria to thrive
2) Water isn’t pure: it will contain dissolved ions like magnesium and calcium
3) Extremely expensive (mainly transport cost)

Note: Tap water is regulated for quality but will often contain relatively high levels of nitrates left over from farms

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29
Q

What are some threats to groundwater supply?

A

-Overpumping
-Pollution - unconfinrd aquifers are more at risk from pollution as they are exposed to the surface.

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30
Q

What are three problems to groundwater abstraction?

A
  • Lowering of the water table
  • Subsidence (collapse)
  • Saltwater encroachment (see case studies)
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31
Q

How is groundwater polluted?

A
  • Caused by factories, leaks, and mines
  • Nitrates, pesticides and .microbes from agricultural run off and sewage
  • Hydrocarbons from petrol stations and factories
  • Toxic fluids from landfill waste disposal sites
  • Acid Mine Drainage (AMD) water containing toxic metals such as lead and cadmium.
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32
Q

Once aquifers are polluted why is it virtually impossible to remove the pollution?

A

Because of the complex groundwater flows that flow underground.

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33
Q

What is the potentiometric surface?

A

The maximum height a fountain can reach depending on its source. If the source is high up, a fountain will flow up to that height.

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34
Q

What is drawdown?

A

The difference in height of the water table from the bottom of a cone of depression to the top of the current water table.

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35
Q

Where will springs form?

A

Where the water table is exposed (ei a mountain with the water table being at a higher level above ground)
Where there is a change in material (ei a dyke or impermeable rock)
Where there is a fault

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36
Q

What are 2 ground improvement methods to reduce permeability of rocks underlying a reservoir

A
  1. Geomembrane/geotextile liner (or clay)
  2. Grouting (eg concrete or silicon to fill holes)
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37
Q

What ground improvement method is used to stop water moving through rocks under a dam?

A

Cut off curtain

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38
Q

Out of the following bedding, which ones would cause an unstable or stable dam?
1. Beds dipping upstream
2. Beds dipping downstream
3. Beds horizontal
4. Syncline
5. Anticline

A
  1. Stable
  2. Unstable
  3. Stable
  4. Stable but with leakage through permeable bed (limestone) due to dipping down the up under a dam
  5. Unstable
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39
Q

In the Vajont dam incident what did geologists ignore?

A
  1. Interbedded limestone and clay dipping towards the reservoir
  2. A scar of an ancient landslide above the reservoir
  3. A small landslide that occurred in 1960 while the reservoir was filling
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40
Q

What happened in the Vajont dam incident?

A
  1. Period of heavy rainfall that percolated through the limestone and sat on the bedding plane of clay
  2. A slide plane developed between the clay and limestone
  3. Caused a landslide of 200 million meters squared of limestone to displace the reservoir, causing a flood and killing 2000 people
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41
Q

What are the benefits of dams (Example of the Aswan dam at the river Nile)?

A

-No more flood damage
-Reliable water supply for irrigation allows new desert areas to be farmed
-Commercial fishing in reservoir
-Boat transport across reservoir
-HEP provides 50% of Egypt’s power supply

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42
Q

What are the problems of dams (using the example of the Aswan dam at the river Nile)?

A

-Abu Simbel would have been flooded so this complete archaeological site was moved
-Aquatic ecosystems damaged
-Reservoir will silt up over time
-Delta no longer receives alluvium
-Agricultural floodplain now needs fertilizer
-Smaller Mediterranean shrimp
-Delta shrinking
-Schistomiasis

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43
Q

How do dams react to earthquakes?

A

-Weight of dam adds stress to underlying rocks
-Weight of water adds stress to underlying rocks
-Reservoir may be over/near existing faults
-Depth of water increases hydrostatic pressure in underlying rocks, which then lubricates faults and bedding planes by increasing pore pressure (overcoming friction)

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44
Q

What is the concentration factor?

A

Is the number of times a metal is concentrated above the average crustal abundance

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45
Q

What is the equation for the concentration factor?

A

CoF = Cut of grade / average crustal abundance

CoF = CoG over Crab

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46
Q

What does grade mean in metals?

A

The % of metal in an ore

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47
Q

What does the cut off grade mean?

A

is the minimum % of metal for an ore (a rock which contains the metal you want) to be economic to mine

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48
Q

Why may the cut off grade and concentration factor change over time?

A

-Supply may go up or down (new discoveries)
-Demand (up/down)
-Other methods of extracting (melting point, solubility, magnetism, density etc)

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49
Q

What are hydrothermal veins and how do they form ores?

A

-Hot water veins that form in late stages of cooling of silicic intrusions (granite/granodiorite)
-Hot water containing dissolved metal sulphides (which are immiscible with the silicate minerals making up the granite/granodiorite)
-Water moves out along joints into cooler country rocks
-Cools and minerals are precipitated to form minerals containing gangue (calcite, quartz, fluorite, barite) plus ore minerals

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50
Q

What is the order in which ores form from intrusion outwards in hydrothermal veins?

A

1 = closest to the granite intrusion
4 = furthest away to the granite intrusion

1 - Tin
2 - Copper
3 - Lead
4 - Zinc

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51
Q

What is the ore mineral of copper?

A

Chalcopyrite

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52
Q

What is the formula for chalcopyrite?

A

CuFeS2

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53
Q

What is the colour, form, hardness, streak, density and cleavage of chalcopyrite?

A

Colour - Brassy
Form - Tetragonal
Hardness - 3.5-4
Streak - Green/black
Density - 4.2
Cleavage - None

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54
Q

What is the formula for gold?

A

Au

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55
Q

What is the colour, form, hardness, density and cleavage of gold?

A

Colour - yellow
Form - Cubic
Hardness - 3
Density - 19.3
Cleavage - None

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56
Q

What is the ore mineral of iron?

A

Magnetite

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57
Q

What is the formula for magnetite?

A

Fe3O4

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58
Q

What is the colour, form, hardness, streak, density and cleavage of magnetite?

A

Colour - Black
Form - Cubic
Hardness - 6
Streak - black
Density - 5.2
Cleavage - poor

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59
Q

What is the ore mineral of lead?

A

Galena

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60
Q

What is the formula for galena?

A

PbS

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61
Q

What is the colour, form, hardness, streak, density and cleavage of galena?

A

Colour - grey
Form - cubic
Hardness - 2.5
Streak - grey
Density - 7.5
Cleavage - 3 at 90 degrees

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62
Q

What is the ore mineral of tin?

A

Cassiterite

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63
Q

What is the formula for Cassiterite

A

SnO2

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64
Q

What is the colour, form, hardness, streak, density and cleavage of cassiterite?

A

Colour - Brown
Form - Tetragonal
Hardness - 6-7
Streak - brown
Density - 7
cleavage - poor

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65
Q

What is the ore mineral of zinc?

A

Sphalerite

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66
Q

What is the formula for Sphalerite?

A

ZnS

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67
Q

What is the colour, form, hardness, streak, density and cleavage of Sphalerite?

A

Colour - Brown
Form - Cubic
Hardness - 3.5-4
Streak - brown
Density - 4.1
cleavage - 6 at 60 degrees

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68
Q

What is ore?

A

The rock containing valuable metals that is economic to mine

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69
Q

What is an ore deposit?

A

An accumulation of metal that may be economic to mine

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70
Q

What is an ore mineral?

A

A mineral containing valuable metals

71
Q

What is gangue?

A

A low-value waste mineral

72
Q

How does secondary enrichment of copper occur?

A
  1. Hydrothermal vein is exposed, with chalcopyrite being evenly spread initially inside
  2. Leaching occurs from rain falling on the exposed hydrothermal vein which percolates downwards in the vein
  3. Above the water table oxidising conditions occur, where chemical reactions change insoluble chalcopyrite into soluble copper sulphates which are carried downwards by groundwater. Malachite and Azurite are precipitated in oxidising conditions as well
  4. A barren, leached zone is left near the surface covered by an insoluble iron oxide capping called gossan
  5. The chalcopyrite is carried downwards to reducing conditions where it is then turned back into chalcopyrite
  6. This means that chalcopyrite is then concentrated below the water table and above the primary copper ore deposit
73
Q

What is the Bingham Canyon?

A

-Large mine
-Copper, silver and gold
- >15 million tons of copper mined
- “Porphyry” deposit: hydrothermal veins from a large granite intrusion (at a destructive plate margin)
-Water (from subducted plate) and metal and heat creates hydrothermal veins
-Sulphides and silicates are immiscible, so copper minerals (chalcopyrite) stayed separate from magma

74
Q

What are placer deposits?

A

A type of mineral deposit in which grains of a valuable mineral like gold or the rare earths are mixed with sand deposited by a river or glacier.

75
Q

What are some minerals that are found in placer deposits?

A

Dense, hard with little cleavage and unreactive/insoluble minerals such as
Cassiterite, gold and diamonds

76
Q

What are typical placer deposit locations?

A

-Confluences
-Waterfall
-Projection of rock / barriers
-Meander (inside bends)
-Beaches and raised beaches

77
Q

What are advantages of dredging and hydraulic mining?

A

-Easy to access to loose sediments
-Cheap to mine (by dredging or hydraulic mining)
-Ores already concentrated
-Lower environmental impact than underground mining

78
Q

What are the disadvantages of dredging and hydraulic mining?

A

-Scars landscape
-Noise
-Dust
-Water pollution
-Placer deposits tend to be small so are quickly exhausted

79
Q

What are four geophysical techniques used to find metals?

A
  1. Gravity surveys
  2. Magnetic surveys
  3. Electromagnetic surveys
  4. Electrical resistivity surveys
80
Q

What are four geochemical surveys used to find metals?

A
  1. Stream sediment sampling
  2. Soil sampling
  3. Water sampling
  4. Vegetation sampling
81
Q

How are magnetic surveys used to find metals?

A

-Magnetometer used to measure small variations in the Earth’s magnetic field strength
-Minerals rich in iron produce an anomaly
-Mafic intrusion containing ore deposits will produce an anomaly
-The presence of the mineral magnetite will produce an anomaly

82
Q

How are electromagnetic surveys used to find metals?

A

-Measures ground conductivity by the process of induction
-EM waves produced by a transmitter induces currents in conducting material such as metallic ore minerals

83
Q

How are electrical resistivity surveys used to find metals?

A

-Two electrodes are placed in the ground and an electric current is passed between them. If the underling rock is a good conductor it will have a low resistance
-If presence of metals, it will have a very low resistance

84
Q

How is stream sediment sampling used to find metals?

A

-Samples of the sediments in a stream are taken and analysed where the interpretation is based on:
1. Stream sediments downstream of the source will have anomalous metal values
2. Stream sediments upstream of the source will have normal metal values
3. The size of the anomaly decreases downstream due to dilution by surface water run off and sediment entering the river
4. Catastrophic dilution occurs where tributaries meet

85
Q

How is soil sampling used to find metals?

A

-Samples of soil are taken and analysed in a grid pattern and analysed where the interpretation is based on:
1. Soil samples on top and downslope of the source will have anomalous metal values
2. Soil samples upslope of the source will have normal metal values

86
Q

How is water sampling used to find metals?

A
  • Either surface or ground water can be collected and tested
    -Normally, there is little traces of metals but if near a mineral deposit it will be higher than usual
87
Q

How is vegetation sampling used to find metals?

A

-Metal traces can be taken up by plants, samples of leaves, twigs or seeds are collected.
- Elevated metal content may indicate the presence of underlying metallic mineral deposits nearby (biogeochemical survey)
-Or geobotanical survey which involves mapping distribution of an indicator plant species that can tolerate high concentrations of metal in the soil

88
Q

How are gravity surveys used to find metals?

A

-Use of gravimeter measured in milligals (mGal)
-Transects and maps
-A positive anomaly = high density mafic/ultra mafic intrusion, containing ore deposits formed by magmatic differentiation and the presence of dense metallic ore minerals)
-A negative anomaly = low density silicic intrusion, which may have hydrothermal mineral veins around it

89
Q

What are 3 mining methods?

A
  1. Above ground: Open cast
  2. Underground: Longwall retreat
  3. Underground: Stope mining
90
Q

What is open cast mining?

A

-Removal of the overburden
-Sediment, blasted rock or ore can then be removed using:
1.Dragline excavator
2. Grab bucket
3. Hydraulic dredging
4. Diggers

91
Q

What are the advantages of open cast mining?

A
  1. Cheap
  2. Safe for workers
  3. Fast
92
Q

What are the disadvantages of open cast mining?

A
  1. Noisy
  2. Visual pollution
  3. Dust (needs to use water sprays)
  4. Potential for environmental damage
93
Q

What is longwall retreat?

A

-Work in blocks and “roads” (grid of tunnels)
-Mechanised: cutter-shearer, conveyer belt and hydraulic roof supports
-Starts furthest away from access shaft
-Cuts a “long-wall”
-Retreats back towards the access shaft
-Allows roof to collapse behind the roof supports

94
Q

What are the advantages of longwall retreat?

A
  1. Very cost-efficient because no ore/rock/coal is left
  2. Few staff underground is all mechanised
95
Q

What are the disadvantages of longwall retreat?

A
  1. Very expensive to set up
  2. If roof collapses it can lead to surface subsidence
96
Q

Where is stope mining usually used?

A

-Usually is used on steeply dipping hydrothermal veins eg tin copper lead and zinc
-Uses explosives and gravity

97
Q

How does stope mining work?

A
  1. Identify mineral vein
  2. Shaft is constructed downwards parallel to the vein
  3. Tunnel is then created inside the bottom of the vein
  4. Explosives are planted in the roof of the vein
  5. Broken ore falls, and is removed using the tunnels created before
  6. Next tunnel is created higher up
  7. Repeat from 4 until all of the vein has been mined or is cost efficient to do so
98
Q

How much ore is needed to be remove to make space for the next tunnel when stope mining?

A

40%

99
Q

What are the 5 stages if the life cycle of a mine?

A

Stage one: Design and planning
Stage two: Construction
Stage three: Production
Stage four: Processing
Stage five: Rehabilitation and reclamation

100
Q

What does the first stage of the life cycle of a mine consist of (design and planning)?

A

-If surveys are promising, this stage will assess if the project will be economically and environmentally viable
-Will see if it is socially responsible as well
-Plan the rehabilitation and reclamation here as well

101
Q

What does the second stage of the life cycle of a mine consist of (Construction)?

A

-Construction includes extraction and processing plants, roads, environmental management systems, housing etc

102
Q

What does the third stage of the life cycle of a mine consist of (Production)?

A

-May be by open cast or underground depending on depth, health and safety, technology, environmental and economic concerns
-Mining starts via mining techniques mentioned before

103
Q

What does the fourth stage of the life cycle of a mine consist of (Processing)?

A

-Mined materials sent through huge crushers to separate valuable ores from gangue
-Will be processed in different ways depending on ore type etc

104
Q

What does the fifth stage of the life cycle of a mine consist of (Rehabilitation and reclamation)?

A

-Once a mine has been exhausted the site is dismantled and land is returned to its original state
-Ensure public safety, environmental quality etc

105
Q

After conducting geophysical surveys, what is used to indicate the areas that are worth spending more time investigating?

A

Target selection using
-GIS software
-Regional geophysical surveys
-Surface mapping, collection via trenching
-Geochemical analysis (chemical, metallurgical, x-ray, spectrographic, radiographic

106
Q

What is exploration drilling and what is it used for?

A

Is used to find the ores present, tonnage, grade, gangue quantity, depth, extent and access of the metal deposit
IS VERY EXPENSIVE TO DO

107
Q

What are the three types of drills used in exploration drilling?

A

Rotary -Leaves small chunks that needs microscopic analysis
Percussion - Hammering into the ground
Core drill (diamond) - Takes core samples

108
Q

Why is estimating the reserve of an ore deposit important and what is used to do so?

A

Is important in seeing if it is economically viable to mine (valuation of the deposit)
Can answer if it is a metal ore body or a mineral deposit

109
Q

What is a geophysical anomaly?

A

A departure from the normal value and may be positive or negative

110
Q

What is a geochemical anomaly?

A

A concentration of a metal above its normal background value

111
Q

What is dispersion?

A

Occurs when small amounts of metals are spread out around the ore deposit by the surface processes of weathering, erosion and transport

112
Q

What is catastrophic dilution?

A

Occurs where tributaries meet and water and sediment from other sources are added

113
Q

Why is finding the grade of an ore body important and how is it done?

A

The ore body may have varying grade, which is done by using statistics such as Kriging
The shape of the ore body may differ so 3-d computer models are used to divide the body into equal sized “blocks” which then are assessed for grade

114
Q

What geological considerations will need to be considered when developing metal deposits using computer modelling?

A

-Geological setting
-Any high grade areas included (The “nugget” effect)
-Styles and zonation of mineralisation (Tin-Copper-Lead-Zinc)
-If mineralisation has a sharp or gradational boundaries
-Presence of toxic elements

115
Q

What does profitability depend on when planning to mine metal deposits?

A

-Type of mine (open/underground)
-Methods and rates of extraction and processing
-Pollution management and AMD (acid mine drainage)
-Dilution by waste rock mixed with ore
-Water control, drainage, pumping etc

116
Q

What economic considerations are needed when looking at the possibility of mining a metal deposit?

A
  • A reserve must have reasonable prospects of economic extraction
    -Set-up costs including assembling plant
    -Cut-off grade
    -Long term projections for metal prices
    -Closure and reclamation plans
117
Q

What factors can affect the accuracy of estimates of metal deposit reserves

A

-Detail of survey
-Variation in grade within the ore body
-Unexpected geological problems (eg faults)
-Unrecorded earlier workings
-Variations in demand
-Improvements in extraction technology

118
Q

What are 4 ways of mineral processing?

A
  1. In-situ leaching
  2. Heap leaching
  3. Froth flotation
  4. Smelting
119
Q

What is in-situ leaching?

A

-Chemical solutions dissolve ore from deep rock, then are re-used
-Drill boreholes, frack then pump leachate down
-“Pregnant” solution is pumped back up (with the mineral inside the leachate)
-Usually used on Uranium and copper especially malachite and azurite

120
Q

What is heap leaching?

A

-Ore is crushed into a heap onto impermeable clay or plastic
-Leaching fluid is then sprayed or leached onto the fluid that percolates through the heap
-The pregnant solution then collects in a pond
-Is cheap but only recovers 60-70% of the desired metal
-Takes anywhere from 2 months to 2 years

121
Q

What is froth floatation?

A

-It separates hydrophobic metals (sulphides) from hydrophilic gangue (quartz)
-Ore is crushed, and water is added to make a slurry
-Chemical is added to make the target mineral hydrophobic
-It is then agitated with bubbles and the froth will then contain the metals
-The tailings are left behind
-Skim the metals off the top

122
Q

What are tailings?

A

Is the gangue and not wanted chemicals
-Will contain toxic metals and toxic chemicals used for leaching and flotation

123
Q

What is the problem of tailings?

A

-storage issues
-are toxic and sometimes radioactive (if uranium)

124
Q

How is smelting used to process minerals?

A

-Extracts elemental metal from ore
-Iron is extracted form hematite in a blast furnace
-Works by the carbon reacting with the oxygen in ores, to leave the metal behind and co2 is then emitted

125
Q

What is mine water, how is it created and what are the problems with it?

A

-(Acid Mine drainage)
-Is the water that will contain dissolved metals in
-Sulphides are mined, cavities are created, groundwater flow is altered, bacteria mixes with water, oxygen and sulphides to create sulphur dioxide which mixes with the water to make AMD (Acid Mine Drainage)
-Wheal Jane - water leaked out into the environment

126
Q

How is AMD managed?

A

-Treat the AMD wherever it flows out of the ground, by neutralising it with base and the precipitated metals will be non toxic salts and will be disposed
-Active treatment, where bases are continuously added such as crushed limestone but is expensive and only really used in large scale mines
-Passive treatment, where wetlands are filled with limestone gravel, and natural oxidation will precipitate metal salts
-Expensive to create but no maintenance, no chemicals or electricity

127
Q

What do hydrocarbon “plays” consist of?

A
  1. Source rock
  2. Maturation
  3. Migration
  4. Reservoir rock
  5. Trap
  6. Cap rock
128
Q

At what temperature do oil and gas form

A

50-150 degrees

129
Q

How does oil and gas form?

A
  1. Plankton in a shallow marine basin
  2. They die, sink and are buried without decaying, to create a carbon-rich rock such as BLACK SHALE
  3. This is the source rock
  4. The organic remains form a material called SAPROPEL
  5. The black shales (and the sapropel) are buried further
  6. SAPROPEL is converted to KEROGEN
  7. As the temperature and pressure rise, MATURATION occurs between 50 and 150 degrees turning Kerogen into PETROLEUM
130
Q

Hoe does oil and gas migrate?

A
  1. Petroleum is less dense than water so rise above water
  2. Oil and gas migrate UP-DIP until they are trapped by a cap rock, or escape from the ground
131
Q

What are 3 types of structural oil and gas traps?

A
  1. Anticline trap
  2. Fault trap
  3. Salt dome trap
132
Q

What are traps?

A

Can be structural or stratigraphic

133
Q

What are 3 types of stratigraphic oil and gas traps?

A
  1. Unconformity trap
  2. Reef trap
  3. Pinch out trap
134
Q

How can petroleum escape or be destroyed?

A

Escape by erosion of cap rock, or up a fault
Can be destroyed by metamorphism, deep burial and igneous activity

135
Q

What is the difference between reserves and resources?

A

Resource - Any useful substance
Reserve - Resources that can be profitably extracted

136
Q

What are blow outs?

A

-Pressure forces oil or gas up a borehole
-Happens during drilling
-Catches fire
-Pollution incidents

137
Q

What is MUD?

A

Drilling fluid that is pumped down boreholes. It is a high density liquid that helps remove rock fragments and keeps drill bits clean. It also maintains hydrostatic pressure (stops rocks leaking into the borehole causing blow-outs)

138
Q

How is oil and gas recovered?

A
  1. Primary recovery (all gas, some oil)
  2. Secondary recovery
139
Q

What is primary recovery?

A

-Multiple wellheads - can dig anywhere
-Under initial pressure gases coming out of solution: expansion of gas above oil; hydrostatic pressure of water below oil
-Once initial pressure drops, use nodding donkeys (beam pumps) - vacuum pumps
-Only recovers 25% due to porosity and permeability of reservoir rock and viscosity of the oil

140
Q

What is secondary recovery?

A

-Aims to extract more oil from reservoir rocks
-Water flood drive = water pumped down to encourage oil to come up another well
-Gas cap drive = pumping gas down, oil will come up another well. Gas can get into pores better than water. It wont work in every situation. Only pushing oil sideways as gas lays on top of oil.
- Steam injection = Same as the other two but loosens up and lowers viscosity of the oil
-Detergents and other chemicals can reduce surface tension of the oil
-Bacteriological methods to break down large hydrocarbons making them less viscous as well
-Still leaves 20-30% of oil

141
Q

Why is some oil difficult to extract?

A

-Very deep = friction increases, and pumping mud becomes harder
-Under deep water = anchoring the floating rig is difficult
-Very viscous = wont flow easily
-Is in an impermeable rock = wont flow easily
-Small oilfields cost too much money to exploit so wont be profitable.

142
Q

What are 6 uses for oil and gas?

A

-Heat source
-Plastics
-Paints
-Perfumes
-Insecticides
-Fertilisers

143
Q

What are two geophysical surveys used to find hydrocarbons?

A
  1. Seismic reflections
  2. Gravity surveys
144
Q

What are two exploration drilling techniques used to find hydrocarbons?

A
  1. Mud logger
  2. Down-hole drilling
145
Q

What are seismic reflections and how are they used to find hydrocarbons?

A

Water based - air gun used to create vibrations, hydrophones detect the reflected waves

Land based - Vibrator trucks - heavy thumper dropped to create vibrations, geophones detect the reflected waves

146
Q

What are gravity surveys and how are they used to find hydrocarbons?

A

-Gravimeters measu8res small variations in gravitational filed strength(in milligals)
-Data is corrected for the effects of latitude and altitude and topography (bouguer correction). Remaining variations are due to densities of rock types beneath (bouguer anomalies)
-Positive anomaly = extra mass such as anticlines or horsts, negative anomalies = deficit of mass, such as a salt dome.

147
Q

What are mud loggers?

A

-Drilling fluid (“mud”) brings 1mm rock chippings up
-The mud logger can then correlate geological sequence with other data
-Chippings are sieved, washed and examined with a microscope and can look at rock type and microfossils
-If the geological sequence is similar to a basin or trap, possibly petroleum

148
Q

What is down-hole logging?

A

-Known as wireline survey
-Geophysical instruments are mounted in a sonde.
-The sonde is pushed into a borehole on a wireline
-As it slowly is pulled back out it measures data
-Porosity
-Gamma ray spectroscopy = source rocks have high gamma arrays emitted
-Resistivity (electricity) = hydrocarbons have high resistivity

149
Q

How do you calculate reserves?

A

Time taken = volume of reserve/production rate

150
Q

How did petroleum form in the North Sea?

A
  1. Europe and North America were joined in the pre-Jurassic
  2. Early rifting was along the line where the North Sea now is
  3. The rift/graben is y-shaped and between Scotland and Norway
  4. The rift flooded and deep deposits of kimmeridgian clay built up. Plankton blooms created a carbon-rich source rock
  5. Then the rift became infilled by a huge delta (mainly porous permeable Brent sandstone)
  6. Burial and maturation of the Kimmeridge clay - produced oil and gas which migrated up-dip into the Brent sandstone reservoir rock
  7. Faulting plus salt mobilisation created lots of trap structures
151
Q

How did petroleum form in the Southern North Sea?

A
  1. Unconventional petroleum
  2. Carboniferous coal (delta-top swamps)
  3. Burial under Permian desert sandstones (Rotliegend sandstone = reservoir rock)
  4. Then covered by Zechstein salt (=cap rock)
152
Q

What are four more unconventional petroleum’s?

A
  1. Oil shale - shale containing lots of kerogen (kimmeridgian clay)
  2. Athabasca tar sands - covers an area of 141,000 km squared of boreal forest and muskeg bog, 165 billion barrel reserve.
  3. Orinoco oil belt of Venezuela - Contains 90% of the global reserve, heavy crude oil.
  4. Frozen gas hydrates - present in ocean floor sediments and permafrost so could also be a future energy source, produce large amounts of methane when melted.
153
Q

What are 4 types of advanced drilling?

A
  1. Horizontal - gains vertical then horizontal into reservoir
  2. Complex path - Access several traps with one borehole
  3. Multiple well - many wells from one well-head, wide area covered
  4. Extended reach - over 10km away eg towns
154
Q

How does fracking work?

A
  1. Drilling - drill hole
  2. Casing - lining of the well
  3. Perfing - Create holes in rock
  4. Fracking - pump high pressure fluid into the cracks
  5. Recovery - recover oil
155
Q

What is the reservoir and sour rock in Wytch farm?

A

Reservoir rock = Jurassic Bird port sandstone, Triassic Sherwood sandstone
Source rock - Lower Jurassic Blue Lias Clay

156
Q

What is the annual production, probable lifespan, and number of wells in Wytch farm?

A

Annual production = 7,300,000 per year
Probable lifespan = until 2037
Number of wells = 103 wells

157
Q

What environmental factors did Wytch farm undergo?

A

-Ecological surveys
-Hidden to the eye
-Area of outstanding natural beauty
-Monitoring air and ground water quality.

158
Q

What are two ways of making tunnels?

A

Cut and shut - trench is dug and a roof is put on it
Immersed tube - Sea bed is dug out, tube immersed in water then filled with sediment

159
Q

What are the advantages and disadvantages of making tunnels in metamorphic and igneous rocks?

A

Advantages - Strong so may not need support
Disadvantages - Drill and blast is used which is slow and expensive, over break and under break are risks (too much or too little explosive)

160
Q

What are the problems that can occur when creating tunnels?

A
  1. Very soft - collapse, leakage eg clay and shale
  2. Strength variation - lateral variation/change of rock type, weathering
  3. Permeable - flooding
  4. Drip - may cross different beds, bedding planes can slip
  5. Faults - weak, fault breccia and gauge, permeable, movement
  6. Planar weaknesses - weak (slippage), permeable, loose blocks
  7. Folds - changes in dip, slippage on bedding planes
  8. Groundwater - free flowing in unconsolidated sediments, strong flows along joints in limestone, high pre fluid pressure in sandstone, saturated clays can move and slump
161
Q

What are the advantages and disadvantages of making tunnels in sedimentary rocks?

A

Advantages - Cheap and easy to dig through using a boring machine
Disadvantages - could collapse so needs lining - use concrete segments and steel ribs

162
Q

What are 5 ground improvement methods for stabilising tunnels?

A
  1. Grouting - fill cracks
  2. Rock drains - drain groundwater
  3. Steel ribs - adds rigidity
  4. Rock bolts
  5. Mesh and shotcrete
163
Q

What does inert mean?

A

Chemically inactive

164
Q

What does bioavailability mean?

A

The proportion or percentage of total metals that are available for incorporation into biota (the plant and animal life), varies with pH

165
Q

What is bioaccumulation?

A

Organisms absorb and store a substance faster than it can be excreted

166
Q

What is biomagnification?

A

Toxin concentration increases up the food chain eg Minamata disease - mercury

167
Q

What is ion exchange?

A

Process used to purify or decontaminate a solution that contains ions

168
Q

What is isomorphous substitution?

A

Replacement, atom by atom, as a crystal grows. Ions must be similar size (less than 15% difference in their ionic radii)

169
Q

What is phytoremediation?

A

Plants deal with the pollution

170
Q

How is waste dealt with?

A

Landfills - waste is put into cells underground lined with a geomembrane (usually bentonite or clays), a tube is put in so methane can escape from the decomposition , and a clay cap is put on top as well.
Groundwater wells monitor the groundwater nearby to see if there is any leakage.

171
Q

What are the environmental problems of nuclear waste?

A
  1. Needs impermeable, aseismic, non-volcanic, above water table, no hurricanes or floods in the area
  2. Thermally hot, coolant will be made radioactive. Natural disaster pose a risk during processing and transport.
  3. > 250,000 years “future proofing”, landscapes erode, sea levels rise, plates move so waste may be exposed
172
Q

What are safety problems with nuclear waste?

A
  1. Vitrification - stability, metal containers are needed but can corrode
  2. Terrorism - dirty bombs, storage sites will be a target, links to military uses
  3. > 250,000 years future civilisations may not know/understand risks of the waste or where it is stored
173
Q

What is carbon capture?

A

-CO2 is a major greenhouse gas and could be collected at source and store underground in salt cavers, oil and gas reservoir rocks and as carbonates. Involves: capture, compress, more in pipeline, inject into rocks.
Is basically sequestration

OCR A LEVEL GEOLOGY (12 decks)

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