http://eps.berkeley.edu/courses/eps50/documents/lecture31.mineralresources.pdf
http://eps.berkeley.edu/courses/eps50/documents/lecture31.mineralresources.pdf
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 environments
• Sedimentary
– Placer – weathering of primary mineralization
and transport by streams (Gold, diamonds,
other)
– Banded Iron Formations – 90%+ of world’s 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)
Hydrothermal Ore Deposits
• Thermal gradients induce convection of
water – leaching, redox rxns, and cooling
create economic mineralization
Black smoker metal precipitation
http://oceanexplorer.noaa.gov/explorations/02fire/background/hirez/chemistry-
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
Metal Sulfide Mineral Solubility
• Problem 1: Transport of Zn to ‘trap’:
ZnS + 2 H+ + 0.5 O2 = Zn2+ + S2- + H2O
log K  9.57  log
[ Zn 2 ] f S 2 [ H 2 O]
[ H  ]2 f O02.5 [ ZnS ]
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 fS2values and compute Zn2+ conc.  Pretty low Zn2+
• 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…
• How about the following:
ZnS + 2 H+ + 0.5 O2 + Cl- = ZnCl+ + S2- + H2O
log K  16.6  log
[ ZnCl  ] f S 2 [ H 2 O]
[ H  ]2 f O02.5 [ ZnS ][Cl  ]
Compared to
log K  9.57  log
[ Zn 2 ] f S 2 [ H 2 O]
[ H  ]2 f O02.5 [ ZnS ]
That is a BIG difference…
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!
Piquette Mine
• 1-5 nm particles of
FeOOH and ZnS –
biogenic precipitation
•Tami collecting
samples
cells
ZnS
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
Fluid Flow and Mineral
Precipitation
• monomineralic if:
– flux Zn2+ > 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
y
Pb2+
ZnS
ZnS
ZnS
PbS
x Zn2+
z HS- generated
by SRB in time t
Model Application
• Use these techniques
to better understand ore
deposit formation and
metal remediation
schemes
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!