Soil

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http//eps.berkeley.edu/courses/eps50/documents/le
cture31.mineralresources.pdf
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http//eps.berkeley.edu/courses/eps50/documents/le
cture31.mineralresources.pdf
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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

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Ore deposit environments

• Sedimentary
• Placer weathering of primary mineralization and
  transport by streams (Gold, diamonds, other)
• Banded Iron Formations 90 of worlds iron
  tied up in these (more later)
• 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)

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Hydrothermal Ore Deposits

• Thermal gradients induce convection of water
  leaching, redox rxns, and cooling create economic
  mineralization

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Black smoker metal precipitation
http//oceanexplorer.noaa.gov/explorations/02fire/
background/hirez/chemistry-hires.jpg
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Water-rock interactions

• To concentrate a material, water must
• Transport the ions
• A trap must cause precipitation in a spatially
  constrained manner
• Trace metals which do not go into igneous
  minerals easily get very concentrated in the last
  bit of melt
• Leaching can preferentially remove materials,
  enriching what is left or having the leachate
  precipitate something further away

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Metal Sulfide Mineral Solubility

• Problem 1 Transport of Zn to trap
• ZnS 2 H 0.5 O2 Zn2 S2- H2O
• Need to determine the redox state the Zn2 would
  have been at equilibrium with
• What other minerals are in the deposit that might
  indicate that? ? define approximate fO2 and fS2-
  values and compute Zn2 conc. ? Pretty low Zn2

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• Must be careful to consider what the conditions
  of water transporting the metals might have been
  ? how can we figure that out??
• What other things might be important in
  increasing the amount of metal a fluid could
  carry? More metal a fluid can hold the quicker a
  larger deposit can be formed

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• How about the following
• ZnS 2 H 0.5 O2 Cl- ZnCl S2- H2O
• Compared to
• That is a BIG difference

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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
• Sulfide (reduced form of S) strongly binds metals
  ? many important metal ore minerals are sulfides!

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Piquette Mine

• 1-5 nm particles of FeOOH and ZnS biogenic
  precipitation

• Tami collecting samples

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cells
ZnS
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Piquette Mine SRB activity

• At low T, thermochemical SO42- reduction is
  WAY TOO SLOW microbes are needed!
• Pure ZnS observed, buffering HS- concentration
  by ZnS precipitation

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Fluid Flow and Mineral Precipitation

• monomineralic if
• flux Zn2 gt HS- generation
• i.e. ? there is always enough Zn2 transported to
  where the HS- is generated, if
• sequential precipitation if
• Zn2 runs out then HS- builds until PbS
  precipitates

z HS- generated by SRB in time t
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Model Application

• Use these techniques to better understand ore
  deposit formation and metal remediation schemes

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Sequential Precipitation Experiments

• SRB cultured in a 125 ml septum flask containing
  equimolar Zn2 and Fe2
• Flask first develops a white precipitate (ZnS)
  and only develops FeS precipitates after most of
  the Zn2 is consumed
• Upcoming work in my lab will investigate this
  process using microelectrodes ? where observation
  of ZnS and FeS molecular clusters will be
  possible!