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

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

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

• 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

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

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 SO4

2- 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

z HS- generated by SRB in time t

x Zn2+

y Pb2+ ZnS

ZnS PbS

ZnS

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!

Hydrothermal Ore Deposits• Thermal gradients induce convection of water –

leaching, redox rxns, and cooling create economic mineralization

• 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