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

• 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

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-hires.jpg

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!