G4 Natural Resources

• • A resource is a concentration of naturally occurring solid, liquid or gaseous material in or on Earth’s crust
  • These can be extracted and processed into useful materials
• • Renewable resources - can be replenished faster or at the same rate than they are being withdrawn
  • Non-renewable resources - not replenished on human time scales, they are being extracted faster than they are being replenished
• • A reserve is the amount of resource that can be extracted at a profit
  • Geochemically abundant - more than 0.1% of Earth’s crust by weight
  • Geochemically scares - less than 0.1% of Earth’s crust by weight
• Ores

  • Form when metal concentrations are above the average crustal abundance
  • They contain a mixture of ore minerals (containing metal) and gangue (low value) minerals
• • The degree to which a metal has been concentrated above average crustal abundance is termed the concentration factor.
  • Concentration factor = concentration of metal in ore / average crustal abundance
• Ore grades

  • The grade is the amount of metal present in a mineral deposit
  • Cut off grade = the minimum amount of metal that is economically viable
  • Cutoff grade = average crustal abundance x minimum concentration factor to be economic
• • Factors causing the cut off grade to be lowered:
  • Increase in value
  • Increase in demand
  • Rare (and useful)
  • Large ore deposit
  • Decreased mining and extraction costs
• • Factors causing the cut off grade to be raised:
  • Decrease in value
  • Decrease in demand
  • Abundant
  • Small ore deposits
  • Increased mining and extraction costs
• • Negative correlation between cutoff grades and reserves. If cut off grades go up, reserves go down.
  • This is because it will be no longer economically viable to mine lower grade deposits.
• An ore forms where metal concentrations are above the average crustal abundance.

  Ores contain a mixture of valuable ore minerals (mineral containing metal) and gangue (low value)

  minerals.

  Examples of gangue minerals include quartz, calcite and pyrite.
• Why are ores important?

  • Metals are scattered unevenly throughout the crust; these are frequently not in sufficiently high enough

  concentrations to mine

  • The degree to which a metal has been concentrated above its average crustal abundance is termed the

  concentration factor

  Concentration factor = concentration of metal in ore (grade) /average crustal abundance
• Ore grades

  The grade is the amount of metal present in a mineral deposit (normally given as a percentage)

  The cut-off grade is the minimum amount of metal that is economically viable

  Cut-off grade = average crustal abundance X minimum concentration factor to be economic
• Current or Wave Action

  Heavy minerals concentrated by flowing water (current or wave action)

  Flowing water carries its load- when energy drops deposition occurs. The heaviest elements are deposited whereas

  the lighter elements continue their journey; gold is concentrated in this manner. These are termed PLACIER

  DEPOSITS.
• • Upstream of projections/undulations on ocean floor or river: projections from the riverbed will trap dense

  placer minerals on the upstream side. E.g. where a dyke juts upwards or on the upstream side of ripples.

  • On beaches- rivers transport sediment into the sea. The sediment may move along the coast by longshore

  drift. Waves thrown sediment up the beach and as the energy of the waves reduces on the backwash, dense

  placer minerals can be left behind forming placer deposits.
• • Flowing meander bends: Placer deposits are found on the inside of meander bends where currents are

  slower (energy lower).

  • Downstream of confluences: where a fast-flowing tributary joins a slower flowing river, the current velocity

  will drop. This results in dense placer minerals being deposited
• Important properties of placer minerals are:

  • Hard, with little or no cleavage: so they survive abrasion and attrition during transport. Gold is an exception.

  Gold is soft but because it is malleable it rolls into nuggets rather than being broken up\

  • Chemically unreactive: so they are not dissolved and taken into solution

  • Dense: so they are deposited first when the current velocity slackens/is reduced
• Chemical Weathering

  Chemical weathering may dissolve other

  minerals and carrying them away in

  solution (leaching), leaving behind a

  concentration of a less soluble valuable

  mineral ore; such concentrations are

  termed RESIDUAL ORE DEPOSITS.
• Bauxite (aluminium ore) is one example and can only be formed by chemical weathering. Despite aluminium being a common mineral it cannot be directly extracted from silicates
• There are several requirements for bauxite to form:

  • Hot and humid tropical climate resulting in intense chemical weathering (high temperatures increase rates of

  chemical reactions and water results in hydrolysis and serves as a catalyst for chemical reactions)

  • Groundwater with a pH between 4 and 10

  • Iron poor and aluminium rich rocks such as granite
• Sometimes chemical weathering results in the leaching of a valuable mineral, carrying it away and concentrating it

  below the water table or when there is a change in the ground conditions. This process is termed SECONDARY

  ENRICHMENT. Copper, for example, can concentrate in such a manner.
• Secondary Enrichment

  • Gossan- upper part of the ore deposit. intensely oxidized, weathered or decomposed rock

  • Rainwater infiltrates the exposed copper deposit.

  • Copper dissolves in solution and is carried downwards

  • At water table conditions change from oxidising to reducing- chemical, reactions change and copper iron

  sulphides are precipitated (chalcopyrite)
• Precipitation

  • Deposition from solution in sedimentary environments

  • Minerals formed by chemical reactions in solution settle out on the sea floor

  • Most important deposits are BANDED IRONSTONES

  • Banded Ironstones formed by oxygen combining with dissolved iron to form insoluble iron oxides.

  • Precipitate forming a thin layer on the ocean floor

  • Termed PRECIPITATED ORE DEPOSITS
• Hydrothermal activity

  • Formed from hot aqueous fluids enriched in minerals

  • Primary source of metals e.g. copper, silver

  • Form hydrothermal ore deposits

  • Silicic intrusions such as granite batholiths are rich in volatiles. Magma is the source of the heat, water and metals.

  • Zoning forms around the intrusion where minerals precipitate out in solubility order. Near the intrusion high temperature and less soluble minerals precipitate (e.g. tin and copper). At lower temps, further from the intrusion, low temperature and more soluble minerals precipitate (e.g. galena and sphalerite).
• Hydrothermal vein formation: Intrusion crystallizes, creating a shell. Hydrothermal fluid, rich in metals, forms at the top. Cooling leads to joint formation, allowing fluid to move into country rock, precipitating ore minerals. Veins form in fractures, showing a symmetrical pattern with concentric zones of ore minerals around the intrusion.
• Differentiation (gravity settling)

  Process where one or more minerals can become locally concentrated (segregated) during the cooling and

  crystallization of magma.

  Chromium (ore is chromite) is an important constituent of stainless steel. One of the first minerals to crystallise out in

  an ultramafic or mafic magma is chromium- this can settle and accumulate at the bottom of the magma chamber into

  dense layers. Magnetite also forms in such a way
• What is a hydrocarbon?

  Organic compound consisting entirely of hydrogen and carbon e.g. oil and natural gas.

  Coal is not a hydrocarbon because although it contains hydrogen and carbon it also contains sulphur and oxygen
• Oil may be defined as carbonaceous matter found in liquid form which has migrated from its site of origin. Oil develops by the physiochemical alteration of accumulating planktonic remains.
• Oil development takes place under these conditions

  • Continuous supply of dead plankton

  • No predators or scavengers

  • Low energy levels to allow build-up of plankton (fine grained sediment represents low energy)

  • Low O2 conditions leading to anoxic conditions

  • Continuous supply of overlying sediments and rapid burial
• Plankton die and sink to the seabed. Plankton buried by the sediments and is converted to SAPROPEL (a dark coloured sediment rich in organic matter). The sapropel is converted to oil shale and bitumen via MATURATION. This takes place at a temperature range between 50-150 degrees C. Beyond 200 degrees C hydrocarbons are destroyed. The temperature change is related to a specific geothermal gradient and so the depth of burial and formation is at a maximum of 7km depth. Conversion from sapropel to oil takes about 1 million years. The speed of conversion determines the grade of the oil.
• Shallow burial produces heavy grade oils (form quickly) while deep burial produces light grade crude oils (form slowly).

  Heavy Grade

  • Relatively young age formed quickly at shallow depths

  Light Grade

  • Formed by deep burial

  • Light because the long chain hydrocarbon molecules are broken into smaller molecules by the increased temperatures and pressures
• Once sapropel has been converted to oil migration of oil follows. This migration distinguishes oil from bitumen and oil shales which are formed in situ. Migration from the source rock is entirely one way and is not reversible. The oil migrates into a reservoir rock which holds it and is prevented from further upward movement by an impermeable cap

  rock.
• Requirements for the formation and accumulation of oil and natural gas

  The main requirements to form economic accumulations are:

  • A source rock

  • Maturation

  • Migration

  • A reservoir rock

  • A cap rock

  • A trap
• SOURCE ROCK

  Burial of plankton in fine grained sediment results in the formation of an organic-rich sedimentary rock called a source rock. Source rocks are black oil shales and mudstones. Their dark colour reflects their high carbon content, and their fine grain size, reflects the low-energy conditions of deposition. The source rocks are commonly clays or shales and are inevitably planktonic rich e.g. Kimmeridge Clay
• MATURATION

  Source rock is buried and subjected to compaction and, due to the geothermal gradient, an increase in temperature. The organic matter breaks down to form a mixture of organic compounds of carbon, hydrogen, nitrogen and sulphur called kerogen and then petroleum. Petroleum forms between temperatures of 50-200 degrees C. Below 50 degrees C biogenic gas forms, but due to its shallow burial, is usually lost. Most oil forms between 50-150 degrees c and natural gas forms between 100-200 degrees C. Above 200 degrees C the hydrocarbons denature and are destroyed.
• MIGRATION

  Once formed the petroleum undergoes migration from the source rock to a reservoir rock. The main factors controlling migration:

  • Permeability of the rocks: there must be permeable rocks between the source rock to a reservoir rock.

  • Pressure: oil and natural gas will migrate down the pressure gradient.

  • Density differences: oil and natural gas are less dense than the water in the pore spaces of rocks, they percolate upwards until they encounter an impermeable layer or reach the surface.

  • Viscosity of the oil: The lower the viscosity of the oil, the more easily it will flow.
• RESERVOIR ROCK

  A reservoir rock must have a high porosity to be able to store significant amounts of oil and natural gas. It must also have a high permeability to allow the oil and natural gas to migrate into it and then be extracted from it. The properties of a reservoir for oil and gas, therefore, need to be the same as for an aquifer for water. Suitable reservoir rocks include poorly cemented sandstones, most limestones and fractured rock.
• CAP ROCK

  The reservoir rock must be overlain by an impermeable cap rock. The cap rock prevents further upwards migration of oil and natural gas. Without a cap rock, the oil and natural gas will continue to rise, eventually forming oil seeps and tar pits on the surface. Suitable cap rocks include fine grained sedimentary rocks such as clay, mudstone and shale and crystalline sedimentary rocks such as evaporates.
• TRAP

  Where the geology allows oil and natural gas to be concentrated in one place, making them economic to extract.
• Anticline trap. Most common oil trap and hold 80% of the worlds natural oil and gas reserves. Oil and natural gas will beconcentrated in the top of the reservoir rock near the crest provided it is overlain by cap rock. The storage capacity of an anticline trap is determined by the size and shape of the fold. An open fold is likely to contain far more oil than a tight fold. The larger the fold the more oil it can store. Once an anticline trap is filled to

  capacity, oil and natural gas leak out laterally, at spill points, migrating into adjacent rocks.
• B. Fault trap. Movement along a fault plane results in a reservoir rock being moved adjacent to an impermeable rock. The impermeable rock on the opposite side of the fault prevents the oil and natural gas escaping laterally. The reservoir rock must also be overlain by a cap rock. Provided the strata are dipping, the oil and gas will migrate up the dip and be trapped at the top of the reservoir rock adjacent to the fault. The fault must be sealed to prevent the oil and gas escaping up the fault.