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Ore deposit environments

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Evaporite deposits minerals like gypsum, halite deposited this way Laterites leaching of rock leaves residual materials behind (Al, Ni, Fe) Supergene ... – PowerPoint PPT presentation

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Title: Ore deposit environments


1
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

2
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

3
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

4
  • 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

5
  • How about the following
  • ZnS 2 H 0.5 O2 Cl- ZnCl S2- H2O
  • Compared to
  • That is a BIG difference

6
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!

7
Piquette Mine
  • 1-5 nm particles of FeOOH and ZnS biogenic
    precipitation
  • Tami collecting samples

8
cells
ZnS
9
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

10
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
11
Model Application
  • Use these techniques to better understand ore
    deposit formation and metal remediation schemes

12
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!

13
Hydrothermal Ore Deposits
  • Thermal gradients induce convection of water
    leaching, redox rxns, and cooling create economic
    mineralization

14
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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