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G4 Earth Resources - Coggle Diagram

- - - - CUMULATES
        
        Formed by slow cooling of large magic and ultra magic intrusions.
        
        minerals with the highest melting points will crystallise first.
        
        the first silicates are FE rich ( olivine). Other non-silicates may have formed before this.
        
        EXAMPLE:
        olivine has a density of 3.2 - 3.5, chromitite is 4.5-4.8.
        Early formed crystals are denser than the melt and begin to sink.
        Any convective motion in the chamber will keep the olivine in suspension.
        Repeated pulses of magma injection will lead to the development of a multi-layered structure at the base of the magma chamber.
      - HYDROTHERMAL MINERAL DEPOSITS
        
        associated with hot waters circulating in the lithosphere with temperatures ranging from 50-400 degrees.
        
        heat source may be magma or an area of the lithosphere rich in radioactive minerals.
        
        Meteoric Water from rainfall, enters rocks by infiltration.
        
        Hydrothermal formation
        
        Hydrothermal mineral veins:
        
        Tin occurs in quartz veins (lodes) as cassiterite along with chalcopyrite. Main or occurs around the edges and roofs of granite bodies emplaced 270Ma - 300 Ma.
        
        Hydrothermal veins originate from intrusion of magma, often a granite batholith, in its later stages of cooling and solidification.
        Incompatible elements which do not fit easily into the atomic structure of the main silicate minerals of the granite may dissolve in the very hot water that can come from rainwater/sea water or from the magma itself. The solution does not mix with the magma- they are immiscible.
        The high pressure (due to depth) prevents the water changing to steam and it can reach temperatures of several hundred degrees.
        
        The pressure forces the hydrothermal solutions into:
        
        joints and fissures in the intrusion formed as it cooled and contracted.
        
        joints and fissures in the country rock formed by the force of the intrusion.
        
        The solutions have a low density and follow the fractures, faults and bedding planes towards the surface.
        Some may permeate through porous rock on either side of the fracture.
        
        As the solution cools, metallic ore minerals (and the associated unwanted gangue minerals) precipitate out in order of solubility:
        
        the less soluble minerals deposited first, nearest the intrusion.e.g Tin
        
        the most soluble ones deposited last, furthest away from the intrusion. e.g lead, iron
        
        Hydrothermal mineralization in Cornwall:
        
        cool groundwaters drawn in and heated to 400 degrees.
        
        hot waters dissolve out trace amounts from a huge volume of granite and the country rock as they circulate through bedding planes, joints and faults.
        
        the granites were emplaced 270 - 300 ma
        
        As the waters move away from the heat source ( granite) they cool and reprecipitate the metals in veins in a more concentrated form
        
        water resources:
        
        meteoric water from rainfall enters rocks by infiltration and percolation.
        
        Seawater contains on average 3.5% dissolved constituents, mainly Na, K, Mg, Ca, Cl and SO4 ions.
        it enters rocks either by infiltration and convection within the sea floor or trapped within marine sediments.
        
        Magmatic water released (exsolved) from magmas during crystalisation can contain as much as 20 - 25% dissolved constituents including metals such as: Cu, Zn, Pb, Fe, Au and Ag.
      - MID OCEAN RIDGE BLACK SMOKERS
        
        mineral rich fluids 275 degrees - 350 degrees meet cold sea water at 2 degrees and precipitate massive sulphide chimneys up to 50m tall.
        
        decompression melting of ultra-mafic mantle = mafic magma
        
        (1) Sea water seeps underground through cracks in the ocean floor along the mid-ocean ridges.
        Once underground, the water is heated to very high temperatures and at the same time dissolving elements from rocks due to heat of the water.
        
        (2) When this super-heated water rises up into the cold ocean, the sudden mixing of hot and cold causes the instantaneous precipitation of the metals and gases suspended in the water.
        Massive amounts of sulphide minerals (metal bearing sulphur compounds), such as pyrite, chalcopyrite, and sphalerite along with silica and the calcium sulphate mineral anhydrite, precipitate out and eventually form towering chimney-like structures.
      - PEGMATITE DEPOSITS
        
        Pegmatites are coarse grained igneous rocks with crystals over 3cm in diameter.
        
        Usually granitic in composition and composed of quartz, k- feldspar and muscovite mica.
        
        often contiain rare minerals such as: Beryllium, uranium, lithium, tin and tungsten.
        
        (1) As crystalization of anhydrous (contains no water) minerals from a granite magma proceeds, the residual fraction becomes progressively enriched in water.
        Rare Earth Element (REEs)ions that are not incorporated into minerals become enriched in this water.
        
        (2) Pegmatites form from this water rich magma - has low viscosity so can grow quickly and to a large size.
        As REEs present - pegmatites can have ecumenic value,
        
        (3) Deposits:
        
        found in fractures along edges of plutons/batholiths and on the edge of the pluton itself.
        
        Pegmatites crystallise from the margin inwards. Anhydrous silicate minerals form the outer zones and hydrous and incompatible rare earth element-bearing minerals form at the centre.
  - - - less resistant minerals worn away or dissolved by water leaving resistant and insoluble minerals as placer deposits.
      - minerals found in placer deposits are:
        
        Hard (>5.0) with little/no cleavage - survive abrasion and attrition. Gold is an exception - soft but malleable so rolls into nuggets and not broken up.
        
        chemically unreactive - not dissolved and taken into solution.
        
        Dense - deposited first when velocity slows (energy drops)
        
        common minerals include : gold. diamond, zircon and platinum.
      - ALLUVIAL DEPOSITS:
        
        At plunge pool/ rapids/ crevices/terraces (old flood plains)- sediment traps in rivers where dense mineral grains can settle to become inaccessible to fast flowing currents.
        
        At stream junctions/confluences - a fast flowing stream slows as it meets a larger more slow stream.
        
        meander bends - inside of meander bend where current slackens and allows selective deposition of heavier mineral grains.
        
        storm beaches - where sorting by wave action leaves behind heavy mineral grains.
        
        raised beaches - where sea level has fallen relative to the land so beach placer deposits may occur inland.
      - IN CORNWALL:
        
        Cassiterite ( tin ) occurs as a hydrothermal deposit.
        
        chemical weathering produces clay from the break down of feldspar and mica, plus unreactive quartz and cassiterite grains.
        
        These weathering products move downslope by mass movement processes.
        
        Weathering products enter stream channels and are transported and sorted according to size/density.
    - - Bauxite (Al):
        the main ore formed as a residual deposit is bauxite, the principle ore for Al.
        
        The crustal abundance of Al is very high (8.1%) but its ore is relatively rare. This is because Al is high in the reactivity series and bonds strongly with other elements that it would be expensive to refine it.
        chemical weathering (carbonation or hydrolysis) performs most of the refining for us.
      - IN JAMAICA: Bauxite occurs in thin layers inter bonded with limestone.
        Jamaica has a hot and wet climate which favours rapid chemical weathering especially of the Tertiary age Limestone.
        
        Bauxite is insoluble and is not affected by the carbonation.
        
        limestone dissolves away, leaving the bauxite to build up ay the surface in situ as a residue.
        
        bauxite can extract Al from the bauxite deposits.
      - China clay/Kaolinite:
        in the clay there is granite which has feldspars.
        feldspar is reactive to hydrolysis which then produces clay.
        
        clay is concentrated where the water is, in the factories of the rock.
    - - Evaporites:
        
        various elements are dispersed throughout sea water in very small amounts, originally derived from land masses by chemical weathering.
        
        Evaporation of the sea water in warm conditions increases the concentration of the salts in the water and they begin to crystalize, according to their solubility (least soluble first)
        SALTS:
        Calite/Aragonite
        gypsum/anhydrite
        halite
        salts of potassium
      - THE BAR THEORY OF EVAPORATE FORMATION:
        
        the shallow lagoon is created by waves crashing over the bar during high spring tides and storms.
        
        the shallow lake just 1-2m deep covers a large area and is known as a playa lake.
        
        the water in the lagoon evaporates to precipitate thin beds of evaporites in the order CGAHK.
        
        3 m of seawater produces just 5 cm of evaporite rock.
        
        many cycles of replenishment, evaporation and subsidence are needed to form thick beds.
      - BANDED IRONSTONE FORMATIONS
        
        Occur in precambrian rocks aged 1,900 - 2750 million years old.
        
        at this time, the earth's atmosphere was deficient in oxygen.
        
        iron released by chemical weathering of magic minerals did not oxidise to ferric (3+) iron but stayed in solution as ferrous (2+) iron.
        
        iron in solution transported to the sea by rivers.
        
        (2) blue- green bacteria had become profuse in shallow seas, producing oxygen by photosynthesis.
        
        in these shallow seas, the free oxygen would locally oxidise the soluble ferrous iron to form insoluble iron oxides and hydroxides as layers on the sea bed.
        
        Banded iron ore comprising alternating layers of haematite, goethite and chert.
        
        (3) not forming at the earth's surface today, uniformitarianism cannot be applied.
        
        iron was precipitated in shallow marine low energy environments as evidenced by ripple marks and mud cracks.
        
        deposits lack classic grains suggesting chemical precipitation far from land or possibly near a low relief land surface with little erosion.
        
        Main ore minerals are hematite and goethite.
        
        (4) some Banded iron formations extend for hundreds of square km and can be up to 500 m thick. some scientists suggests the finest banding may be diurnal.
- - - - Any local variation in gravity observed then depends on:
        
        differences in density of rocks beneath you.
        
        distance between the body and point of measurement (depth)
        
        the size of the body
      - Bouguer Gravity Anomalies:
        
        differences in gravity either higher or lower than the average to be expected for a homogenous warth.
        
        negative anomalies = lower density rocks.
        
        e.g low density silicic intrusion/ low density salt dome.
        
        positive anomalies = higher density rocks
        e.g ultramafic/mafic intrusion which cumulate ore deposits.
        
        dense metallic ore minerals.
        
        Anticline (denser rock from below bought to higher levels).
        
        the deeper a body of unusually high or low density rock, the less obvious is the response at the surface.
        
        a large deep body may produce a similar response to a smaller shallower body.
    - - measured by a magnometer, determines geomagnetic field strength.
        
        can be airborne.
        
        geologists study the variation in magnetic field or the magnetic anomaly.
        
        (1) Minerals rich in Iron, nickel and cobalt produce positive magnetic anomalies. The presence of these could be due to:
        
        A magic/ultramafic intrusion with may contain ore deposits formed by gravity settling.
        
        magnetite present.
        
        finding steel tanks and waste drums.
        
        detecting iron and steel obstructions.
        
        locating unmarked mineshafts.
        
        mapping mafic, igneous intrusive rocks.
    - - Resistivity
        
        two electrodes placed in the ground.
        
        electric current passed through them
        
        two inner electrodes are used to measure the voltage.
        
        from these measure menus the resistance of the underlying rock can be calculated.
        
        (1) good conductors like rocks containing metallic ore minerals will have a lower resistance than other rocks.
        Each rock type has a characteristic range of resistivities.
        If water is present in pore spaces, then this water acts as a conductor.
        Quartz has a high resistivity but a quartz sandstone with water in pore spaces would have a low resistivity.
        
        (2) As resistivities are generally large the current applied cannot travel far so can only measure to a few metres below ground surface.
        other methods involving resistivity measurements in boreholes are used to see deeper.
        As water affects the readings the weather is important. two sets of readings from a wet and dry day may be different.
        
        Measures the water table depth.
        
        Detects solution features and voids.
        
        located buried alluvial channels.
        
        identifies fracture zones and discontinuities.
    - - electromagnetic waves produced by a transmitter induce current in conducting minerals like metallic ore minerals. this current creates a magnetic field that you can measure.
      - Often use EM waves from a very low frequency (VLF) transmitter that governments use for communication/ navigation
    - - shows boundaries between rock types (and thus wave speed)
        the waves either :
        
        reflect off boundary
        
        refract across the boundary
      - you need :
        
        artificial source of ground waves
        
        an array of detectors/geophones
        
        mobile recording equipment
        
        sound wave generation
        
        early days dynamite. today, heavy pad vibrated hydraulically, manually or electromagnetically. At sea, compressed air gun.
        
        sound waves generated
        
        sound waves reflected back to surface from geological boundaries + geophones record the reflected waves.
        
        vibrator truck and the recording truck move down the section line at regular intervals to generate a 2 D seismic reflection profile.
        
        two way travel times are recorded TWT
        
        if measuring depth to sea bed:
        DEPTH = (TIME/2) * SPEED OF WAVE IN WATER
        
        need to know the velocity of the rock (to calculate the depth)
        a rock can give different velocities depending on things like:
        
        amount of water in pore spaces
        
        amount of pressure the rock is under
        
        We are interested in boundaries - can't tell nature of boundary i.e if fault, unconformity, normal bedding changes.
      - P, S (surface) Raleigh, Love waves.
        
        we want the P&S waves that have travelled downwards.
        they travel faster than surface waves so by only measuring the first arrival at the geophones we know we are measuring P, S waves.
  - - - orientation survey- know the geological setting of the ore body you are interested in e.g. associated with granite intrusion.
        Establish a background concentration of element of interest.
        reconnaissance survey: carry out over a large area.
        Detailed survey: focus on area of interest - make sure take readings at closer and closer distances once establish likely location for ore body
        the ore body lies at centre of the anomaly,
    - - BUT measuring small concentrations of metal AND have to take into account seasonal variations in discharge which distort the concentration readings.
    - - stream sediment analysis - panning
        
        traditional technique for finding gold in stream sediments.
        
        several handfuls of sediment are placed in the pan, the pan is immersed in water then gently swirled.
        
        lower density silicates floated off, denser metallic ores such as gold or cassiterite remain in the bottom of the pan. (den over 6)
- - - - factors affecting porosity:
        
        rock type - crystalline rocks usually <1%
        
        degree of sorting:
        well sorted = high porosity = large pore spaces
        poorly sorted = low porosity = fine sediments fill in the pore spaces.
        
        diagenesis: loose and unconsolidated = high porosity
        more compacted and cemented = lower porosity.
        
        grain shape = the more rounded the grains, the higher porosity.
        the more angular grains, the lower porosity.
    - - permeability depends on
        
        connectivity of pores
        
        grain size - coarse grained - higher permeability.
        less resistance (friction) to flow past.
        
        number, size and connectivity of joints and fractures (secondary porosity/permeability).
      - Sandstone: has a lower porosity than clay bit the water can move, so the rock is permeable.
        
        poorly sorted or well cemented sandstones will have a lower porosity. the pore space is filled by fine grained material or cement.
      - Granite: may have a very low porosity, but water can flow through it relatively quickly due to the interconnected cracks and joints.
      - Chalk: is very fine grained limestone and has a high porosity and permeability due to microscopic pores as well as larger cracks and joints.
- - - - plant growth is a function of temperature and precipitation.
        
        warm wet environments generally have the most prolific plant growth.
        
        these conditions common in carboniferous in the uk.
        
        a succession of cyclothems are formed as the delta subsides due to the weight of sediment deposited on top and the sequence restarts.
        or can restart if sea level rises
      - diagenesis turns peat into coal - coalification:
        
        peat compacted by weight of the overturn of sediments.
        
        water and other volatiles are squeezed out.
        
        oxygen lost as CO2 and H2O
        
        Hydrogen lost as methane (this van accumulate as natural gas elsewhere) HAZARD OF MINING COAL.
        
        10m of peat gives 1m of coal.
      - coal composition:
        50% carbon
        volatiles: sulphur, chlorine, phosphorus, nitrogen
        trace amounts: dirt, other elements.
    - - lignite:
        brown colour, some woody material visible, dull colour.
        most polluting
        
        Bituminous:
        black with banding. some shiny parts
        
        Anthracite:
        very hard, black, shiny/metallic, higher density.
        purest and least polluting
  - - - Resevoir rocks are a highly porous and permeable rock capable of storing and yielding significant amounts of petroleum. Sandstones, limestones and fractured chalk.
      - Cap rock is the impermeable rock above the reservoir rock preventing further upward movement of petroleum. Clays, mudstones and evaporites.
    - - STRUCTURAL TRAPS:
        
        Anticline- most common, open fold can contain more than tight fold. once full to capacity, they can leak out laterally.
        
        Fault trap- brings impermeable rock next to a reservoir rock, preventing oil and gas from escaping. fault MUST be sealed to prevent hydrocarbons escaping along it.
        
        Salt dome trap- evaporites have lower density than surrounding rocks so (like magma) form diapers that rise upwards. These uplift surrounding rock to form a dome.
        Dome acts as a cap rock as its impermeable (evaporites). hydrocarbons found in dipping reservoir rocks next to the salt dome OR in the anticline trap created above the salt dome.
      - LITHOLOGICAL TRAPS:
        
        Unconformity trap- need impermeable either side, rocks above unconformity act as the cap rock.
        
        stratigraphic trap- limestone reef; lots of life when formed so organic matter present. Need impermeable rock either side.
        Also delta sandstone often form in lens shapes and make good reservoirs.