water-rock interactions to concentrate a material, water must: –transport the ions –a ‘trap’...
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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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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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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+
][][
][][log57.9log
5.02
2
22
2
ZnSfH
OHfZnK
O
S
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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…
]][[][
][][log6.16log
5.02
2
22
ClZnSfH
OHfZnClK
O
S
][][
][][log57.9log
5.02
2
22
2
ZnSfH
OHfZnK
O
S
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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 SO4
2- 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+ > 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
x Zn2+
y Pb2+ ZnS
ZnS PbS
ZnS
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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!
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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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• 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)
Ore deposit environments
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• Placer uranium gold
• Stratiform phosphate
• Stratiform iron
• Residually enriched deposit
• Evaporites
• Exhalative base metal sulphides
• Unconfornity-associated uranium
• Stratabound clastic-hosted uranium, lead, copper
• Volcanic redbed copper
• Mississippi Valley-type lead-zinc
• Ultramafic-hosted asbestos
• Vein uranium
• Arsenide vein silver, uranium
• Lode Gold
Ore Deposit Types I
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• Clastic metasediment-hosted vein silver-lead-zinc
• Vein Copper
• Vein-stockwork tin, tungsten
• Porphyry copper, gold, molybdenum, tungsten, tin, silver
• Skarn deposits
• Granitic pegmatites
• Kiruna/Olympic Dam-type iron, copper, uranium, gold, silver
• Peralkaline rock-associated rare metals
• Carbonatite-associated deposits
• Primary diamond deposits
• Mafic intrusion-hosted titanium-iron
• Magmatic nickel-copper-platinum group elements
• Mafic/ultramafic-hosted chromite
Ore Deposit Types II