Once again, we will use the mnemonic SADE as an introduction to the formation of sedimentary ore bod

1
Sedimentary Ore Deposits

• Introduction
• Once again, we will use the mnemonic SADE as an
  introduction to the formation of sedimentary ore
  bodies.
• Sources Earths crust, oceans (direct
  extraction and indirect source of evaporites and
  organic material)
• Agent Physical transportation as grains or
  chemical in solution as ions or as colloidal
  particles.
• Deposition In this context they are at or just
  beneath the surface of the Earths crust, both on
  land and under water.
• Energy Suns heat, causing evaporation and
  assisting with chemical reactions (weathering),
  and gravity causing rain.

2
Sedimentary Ore Deposits

• Surface Processes

3
Sedimentary Ore Deposits

• Surface Processes
• Weathering
• Weathering is defined as the in situ breakdown of
  rocks and includes physical processes such as
  frost shattering and chemical processes
  (including biochemical) involving rainwater,
  which break down minerals which are unstable at
  the earths surface, usually by forming more
  stable minerals and taking soluble products into
  solution.
• Erosion
• Removal of exposed rock is called erosion and the
  transportation can take the form of a bedload,
  suspension, colloidal and solution. Colloidal
  particles are essentially insoluble but fail to
  precipitate due to their electrostatic charge, so
  act as soluble.

4
Sedimentary Ore Deposits

• Surface Processes
• Deposition during Physical Transport
• Deposition is determined by the energy of the
  environment. When the energy of the environment
  declines, the larger heavier grains are first to
  be deposited, followed by lighter, smaller
  grains. When these sedimentary deposits contain
  ore mineral grains they are known as placer
  deposits.
• Deposition during Chemical Transport
• Deposition is determined by a change in chemistry
  of the environment, such as its pH or oxidising
  conditions. This can affect both solutions and
  colloidal particles and is important in forming
  precipitated sediments and bedded ore deposits.

5
Sedimentary Ore Deposits

• Ores formed by chemical leaching and deposition
  from groundwaters
• Residual weathering deposits - Laterites
• In tropical climates, where chemical weathering
  is high but erosion I slow, thick soils known as
  laterites develop. They contain the clay mineral
  kaolinite and insoluble hydrated oxides of Al and
  Fe (which makes them deep red). The downward
  movement of percolating water and the upward
  movement of moisture during dry spells, mobilises
  soluble ions in solution and transport fine
  grained particles through the soil. The result
  is enriched clay in the deeper layers and upper
  layers richer in Fe. Laterites developed mainly
  on Fe-rich rocks (peridotite).

6
Sedimentary Ore Deposits

• Residual weathering deposits Laterites cont..
• Pacific Islands of New Caledonia - Nickel
• The Fe content of these laterites is too low to
  be of value, but where the underlying peridotite
  is abundant in Ni, then through leaching and
  deposition at the base of the laterite, deposits
  are formed which make up over 60 of all Ni
  reserves. Co, Cu and Cr are often valuable
  by-products where because of the ease of
  extraction grades of Cr _at_ 3.6 can be economic to
  mine, whereas grades of 30 Cr are needed in
  magmatic chromite deposits.

7
Sedimentary Ore Deposits

• Residual weathering deposits Laterites cont..

8
Sedimentary Ore Deposits

• Residual weathering deposits Laterites cont..
• Bauxite
• The most important metal in quantity is Al which
  is obtained from particular laterites, rich in
  hydrated Al oxides, called bauxite. Al is the
  most abundant metal in the Earths crust, but it
  is usually in the form of silicates which is no
  use. It is formed by the normal chemical
  breakdown of K feldspars to Kaolinite being
  extended by the silica being leached out to form
  insoluble hydrated Al oxides in warm wet
  climates.

2KAlSi3O8 2H H2O
Al2Si2O5(OH)4 2K 4SiO2 potassium
feld acid rainwater kaolinite
ions silica in sol. Al2Si2O5(OH)4 H2O
Al2O3.3H2O
2SiO2 kaolinite insol. hydrated Al oxides
soluble Si
9
Sedimentary Ore Deposits

• Residual weathering deposits Laterites cont..
• Bauxite cont.
• Although Si is more easily dissolved than Al in
  normal surface water solutions, silicate minerals
  still require extreme chemical weathering to
  break down completely. For bauxite to form you
  also require the correct physical requirements of
  tropical temps, intermittent heavy rainfall and
  low erosion, as well as low impurities.
• Granites have suitable chemical composition from
  which bauxites can form, but is not always
  fractured enough to allow easy weathering and has
  sometimes already had its minerals altered by
  hydrothermal fluids.
• Volcanic ash usually has a more open texture,
  allowing easy access to surface waters.

10
Sedimentary Ore Deposits

• Residual weathering deposits Laterites cont..
• Bauxite cont.
• The primary reserves are in Australia, Guinea,
  Equatorial South America (Brazil, Venezuela,
  Guyana), Jamaica and India. Greece and Hungary
  also have some major reserves which have
  developed from volcanic ash of wind-borne dust
  from deserts, coming to rest on permeable
  limestones which make a good trap and allow good
  drainage.

11
Sedimentary Ore Deposits

• Secondary Enrichment Deposits
• Secondary enrichment takes place when the
  dissolved metal ions which have been removed from
  the residual deposits we have just looked at,
  come out of solution
• When rainwater enters soils and rock it is
  oxidising and slightly acidic, but as it
  interacts with organic matter and metal ions,
  oxygen is used up and it becomes more reducing
  with depth.
• E.g. chalcopyrite near the surface will be
  oxidised to form Fe hydroxide and soluble Cu and
  sulphate ions. The Fe hydroxide is a residual
  deposit composed largely of rusty limonite and
  forms an Fe cap or Gossan.
• But what happens to the ions that are in solution
  and pass downwards?

12
Sedimentary Ore Deposits

• Secondary Enrichment Deposits cont..
• Soluble ions
• In the oxidising conditions above the water
  table, some ions may combine with bicarbonate
  (HCO3-) ions from rainwater to form carbonate
  minerals. Some Cu precipitates as malachite,
  azurite or native Cu, but most will stay in
  solution until it reaches the reducing
  environment.
• In this environment, existing sulphide minerals
  (primary ore minerals) do not break down, but can
  react chemically with the incoming metal ions to
  produce secondary ore minerals. The example
  below shows secondary Cu minerals richer in Cu
  than the primary mineral.

CuFeS2 Cu2 2CuS
Fe2 chalcopyrite covellite
13
Sedimentary Ore Deposits

• Secondary Enrichment Deposits cont..
• Soluble ions
• Ag and Zn behave similarly to Cu in that they
  will enter into solution (be leached) from
  minerals where groundwater conditions are
  sulphate-bearing and oxidising.
• As these pass down into reducing sulphide-bearing
  environments, Zn remains fairly soluble, so will
  stay in solution while Cu and Ag will precipitate
  out to form secondary enrichment deposits. This
  can be particularly significant in porphyry Cu
  deposits such as Chuquicamata where secondary
  enrichment locally reaches grades of 15 Cu.

14
Sedimentary Ore Deposits

• Secondary Enrichment Deposits cont..

15
Sedimentary Ore Deposits

• Ores formed through chemical transport and
  deposition by surface waters
• Chemical transport includes dissolved metal ions
  and minute colloidal particles, both of which
  need a change in the chemical environment to be
  deposited. Changes in solubility are caused by
  temperature, pH or oxidising condition of the
  solution. A change which can cause colloidal
  particles to separate out is an increase in
  salinity, neutralising the charges on the
  particles and enabling them to coalesce to form
  larger particles (flocculate) and settle out as a
  chemically formed sediment. This can be
  effective for silica, clay minerals and
  sulphides.
• Flocculation can also include the loose
  attachment of metal ions.

16
Sedimentary Ore Deposits

• Ores formed through chemical transport and
  deposition by surface waters cont..
• A more important mechanism by which metals are
  concentrated from sea water is by absorption on
  to organic material. Accumulation of organic
  matter forms carbonaceous black shales on the
  ocean floor where conditions become very
  reducing. The shales are often rich in trace
  metals (Cu, Zn, Mo, U). They never constitute
  ore deposits but can be and important source of
  metals for other processes.

17
Sedimentary Ore Deposits

• Ores formed through chemical transport and
  deposition by surface waters cont..

18
Sedimentary Ore Deposits

• Ores formed through chemical transport and
  deposition by surface waters cont..
• Bedded Iron Ores
• Concentrations of Fe oxides and carbonates are
  thought to have been formed mainly through
  chemical precipitation from solution. But how
  can this happen when Fe(III) oxides are very
  insoluble in oxidising conditions (therefore
  precipitates easily on the surface) and Fe(II) is
  really only soluble in acidic reducing
  environments.
• One possibility is that precipitation may occur
  where Fe(II) bearing swamp waters overflow into
  active, oxygenated, drainage channels. Fe(II) is
  oxidised, forming insoluble hydrated Fe(III)
  oxide which may precipitate and settle
  immediately or later on.

19
Sedimentary Ore Deposits

• Bedded Iron Ores cont..
• This mechanism can account for the Carboniferous
  Fe stones, which are carbonates and occur
  alongside coal, so were an important factor in
  the early development of the industrial
  revolution. The Jurassic Fe stones consist
  mainly of a hydrous Fe silicate called
  berthierine, which forms ooliths (sand sized
  grains with a concentric layered structure) and
  siderite which forms the matrix between the
  ooliths.
• The mining of these two occurrences of Fe stone
  has now been superseded by deposits which formed
  in Precambrian times, before plants colonised
  land areas. We must therefore be looking at a
  different mode of formation.

20
Sedimentary Ore Deposits

• Bedded Iron Ores cont..
• Banded Iron Formation (BIF)
• Ukraine, Brazil, Australia, Canada.
• The Precambrian Fe stones occur in bands
  alternating with layers of chert (precipitated
  silica). Conditions were very different then,
  with relatively low oxygen levels allowing Fe to
  be transported as Fe2(aq) ions, but algae
  probably assisting in the precipitation of
  Fe(III) oxides.
• There are huge resources of Fe in BIF around the
  world, but only a small proportion of this is
  currently economic with grades of 30-35. Where
  silica has been leached from the layers of chert,
  and haematite and magnetite have recrystallised,
  enrichment up to 60 can take place.

21
Sedimentary Ore Deposits

• Ores formed through chemical transport and
  deposition by surface waters cont..
• Manganese Nodules
• They occur scattered in abundance over the sea
  floor where sedimentation is slow. They are
  composed of hydrated Fe and Mn oxides, built up
  in layers to form a concentric banded nodule.
  They contain considerable Mn, but also some
  valuable trace metals. Takes 10-20Ma for a 10cm
  nodule to grow. They are not currently exploited
  because
• They occur at over 3km depth in the ocean.
• Although they are abundant, they are spread very
  thin.
• Who owns them? Most are outside of 200 mile
  limits.
• Strong environmental considerations.

22
Sedimentary Ore Deposits

• Ores formed through physical transport and
  deposition by surface waters placer deposits
• The energy of the transporting environment
  determines what grains are transported, while any
  change in the energy will sort those grains.
  E.g. the slackening of a current in a high energy
  stream will cause larger and heavier grains to be
  deposited, whereas smaller, lighter particles
  remain in suspension or move as bedload.
• Ore minerals are much denser than other
  rock-forming silicate minerals, so can be
  deposited early and concentrated. They are
  frequently trapped in locations where flow rates
  diminish rapidly forming placer deposits.

23
Sedimentary Ore Deposits

• Placer deposits cont..
• Typical locations of placer deposits in alluvial
  and coastal environments.

24
Sedimentary Ore Deposits

• Placer deposits cont..

25
Sedimentary Ore Deposits

• Placer deposits cont..
• The occurrence of ore minerals in placer deposits
  will depend entirely on the source rocks and the
  efficiency of sorting.
• Most placer deposits occur in loose sediments,
  they are easily worked and the ore minerals are
  easily separated.
• Much of the worlds tin is obtained from Brazil
  and Malaysia using high pressure water jets.
• Beach placer deposits are often mined by
  dredging.
• Placer deposits which have been buried and
  consolidated since their initial accumulation are
  known as fossil placer deposits. The best
  known is the Witwatersrand Goldfield in S. Africa
  which has provided over 50 of the worlds gold
  since mining started at the end of the 19th C.

26
Sedimentary Ore Deposits

• Summary
• Processes at the Earths surface can form bulk
  deposits of industrial minerals and concentrate
  metals to form ore deposits. Resistant minerals
  can be conc. by physical transport to form
  placers. Chemical processes conc. Metals through
  removal of soluble material to leave residual
  products, and by precipitation of that dissolved
  material to form bedded sedimentary products and
  secondary enrichment.
• Chemical transport and precipitation is largely
  controlled by changes in pH or oxidising
  conditions. Near surface is oxidising/slightly
  acidic deeper is higher pH/more reducing.
• Laterites - insoluble residues resulting from
  intense weather-ing in tropical climates, with
  high but intermittent rainfall, good drainage and
  low erosion. Bauxite is most important.

27
Sedimentary Ore Deposits

• Summary
• Secondary enrichment caused by oxidising
  rainwater leaching metals such as Cu Ag from
  sulphide minerals. The solutions migrate down to
  the water table, where they may react with
  primary ore minerals to produced enriched
  minerals
• Chemical (or biochemical) precipitation in
  shallow seas to produce Pre-Cambrian BIF and
  Jurassic Fe stones. Economic grades often
  enhanced by secondary enrichment.
• Manganese nodules form slowly on the ocean floor
  by precipitation of hydrated Fe(III) and Mn(IV).
• Placer deposits are concentrations of hard,
  dense, chemically stable minerals which are
  physically concentrated through transport in
  surface waters and then deposited in rivers and
  along coasts. Fossil placer deposits are
  highly significant.