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Ores. Principally we discuss ores as sources of metals However, there are many other resources bound in minerals which we find useful How many can we think of?. Ore Deposits. A deposit contains an unusually high concentration of particular element(s)

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slide1
Ores
  • Principally we discuss ores as sources of metals
  • However, there are many other resources bound in minerals which we find useful
  • How many can we think of?
ore deposits
Ore Deposits
  • A deposit contains an unusually high concentration of particular element(s)
  • This means the element(s) have been concentrated in a particular area due to some process
  • What sort of processes might concentrate these elements in one place?
gold au
Gold  Au
  • Distribution of Au in the crust = 3.1 ppb by weight  3.1 units gold / 1,000,000,000 units of total crust = 0.00000031% Au
  • Concentration of Au needed to be economically viable as a deposit = few g/t  3 g / 1000kg = 3g/ 1,000,000 g = 0.00031% Au
  • Need to concentrate Au at least 1000-fold to be a viable deposit
  • Rare mines can be up to a few percent gold (extremely high grade)!
ore minerals
Ore minerals
  • Minerals with economic value are ore minerals
  • Minerals often associated with ore minerals but which do not have economic value are gangue minerals
  • Key to economic deposits are geochemical traps  metals are transported and precipitated in a very concentrated fashion
    • Gold is almost 1,000,000 times less abundant than is iron
economic geology
Economic Geology
  • Understanding of how metalliferous minerals become concentrated key to ore deposits…
  • Getting them out at a profit determines where/when they come out
ore deposit environments
Ore deposit environments
  • Magmatic
    • Cumulate deposits – fractional crystallization processes can concentrate metals (Cr, Fe, Pt)
    • Pegmatites – late staged crystallization forms pegmatites and many residual elements are concentrated (Li, Ce, Be, Sn, and U)
  • Hydrothermal
    • Magmatic fluid - directly associated with magma
    • Porphyries - Hot water heated by pluton
    • Skarn – hot water associated with contact metamorphisms
    • Exhalatives – hot water flowing to surface
    • Epigenetic – hot water not directly associated with pluton
ore deposit environments1
Ore deposit environments
  • Sedimentary
    • Placer – weathering of primary minerals and transport by streams (Gold, diamonds, other)
    • Banded Iron Formations – 90%+ of world’s iron tied up in these
    • Evaporite deposits – minerals like gypsum, halite deposited this way
    • Laterites – leaching of rock leaves residual materials behind (Al, Ni, Fe)
    • Supergene – reworking of primary ore deposits remobilizes metals (often over short distances)
geochemical traps
Geochemical Traps
  • Similar to chemical sedimentary rocks – must leach material into fluid, transport and deposit ions as minerals…
  • pH, redox, T changes and mixing of different fluids results in ore mineralization
  • Cause metals to go from soluble to insoluble
  • Sulfides (reduced form of S) strongly binds metals  many important metal ore minerals are sulfides!
  • Oxides – Oxidizing environments form (hydroxy)oxide minerals, very insoluble metal concentrations (especially Fe, Mn, Al)
hydrothermal ore deposits
Hydrothermal Ore Deposits
  • Thermal gradients induce convection of water – leaching, redox rxns, and cooling create economic mineralization
massive sulfide deposits
Massive sulfide deposits
  • Hot, briny, water leaches metals from basaltic ocean rocks
  • Comes in contact with cool ocean water
  • Sulfides precipitate 
vermont copperbelt
Vermont Copperbelt
  • Besshi-type massive sulfide deposits
  • Key Units:
    • Giles Mountain formation – More siliciclastic, including graphitic pelite, quartoze granofels (metamorphosed greywacke), hornblende schist, amphibolite
    • Standing Pond Volcanics – mostly a fine grained hormblende-plagioclase amphibolite, likely formed from extrusive basaltic rocks (local evidence of pillow structures in St. Johnsbury). Felsic dike near Springfiled VT yielded a U-Pb age of 423± 4 Ma.
    • Waits River formation – Calcareous pelite (metamorphosed mudstone), metalimestone, metadolostone, quartzite.
minerals associated with economically recoverable metals
Minerals associated with economically recoverable metals
  • Elemental forms
  • Sulfides
  • Oxides
  • Carbonates
  • Sulfate salt

Cuprite, Cu2O

Elemental copper

Malachite, Cu2CO3(OH)2

Chalcocite, Cu2S

Chalcanthite, CuSO4*5H2O

sulfides part 1
Sulfides Part 1
  • Substitution into sulfides is very common
  • As and Se substitute for S very easily
  • Au can substitute in cation sites (auriferrous minerals)
  • Different metals swap in and out pretty easily  Cu and Fe for instance have a wide range of solid solution materials
sulfide minerals
Sulfide Minerals
  • Minerals with S- or S2- (monosulfides) or S22- (disulfides) as anionic group
  • Transition metals bonded with sulfide anion groups
iron sulfides
Iron Sulfides
  • Mackinawite – FeS
  • Greigite – FexSy
  • Pyrite – FeS2 (cubic)
  • Marcasite – FeS2 (orthorhombic)
  • Troilite – FeS end member
  • Pyrrhotite – Fe1-xS (slightly deficient in iron)
  • Arsenopyrite – FeAsS
  • Chalcopyrite – CuFeS2
other important sulfides
Other important sulfides
  • Galena – PbS
  • Sphalerite/wurtzite – ZnS
  • Cinnabar – HgS
  • Molybdenite – MoS
  • Covellite – CuS
  • Chalcocite – Cu2S
  • Acanthite or Argenite – AgS
  • Stibnite – Sb2S3
  • Orpiment – As2S3 ; Realgar – AsS
sulfides are reduced minerals what happens when they contact o 2
Sulfides are reduced minerals  what happens when they contact O2?
  • This is the basis for supergene enrichment and acidic mine drainage
actively oxidizing pyrite
Actively Oxidizing Pyrite
  • FeS2 + 3.5 O2 + H2O  Fe2+ + 2 SO42- + 2 H+
  • FeS2 + 14 Fe3+ + 8 H2O  15 Fe2+ + 2 SO42- + 16 H+
  • 14Fe2+ + 3.5 O2 + 14H+ 14 Fe3+ + 7 H2O
  • Sulfur species and H+ generation:
    • FeS2 + 2 Fe3+à 3 Fe2+ + ¼ S8 + 0 H+
    • FeS2 + 7 Fe3+ + 3 H2Oà 8 Fe2+ + 0.5 S4O62- + 6 H+
amd neutralization
AMD neutralization
  • Metals are soluble in low pH solutions – can get 100’s of grams of metal into a liter of very acidic solution
  • HOWEVER – eventually that solution will get neutralized (reaction with other rocks, CO2 in the atmosphere, etc.) and the metals are not so soluble  but oxidized S (sulfate, SO42-) is very soluble
  • A different kind of mineral is formed!
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