Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Hydrothermal Solutions andOre Deposits

Physical Chemistry of Minerals and AqueousSolutions

D.M. Sherman, University of Bristol

Chalcophiles, Lithophiles, Siderophiles..

Lithophile = oxides, silicates

Siderophile = Fe alloys

Chalcophile = sulfides

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Hydrothermal Vein Deposits

Hypothermal(300-600 oC)

Mesothermal(200-300 oC)

Epithermal(50-200 oC)

Sulfide Ore Minerals

Molybdenite MoS2

Pyrrhotite Fe1-xSChalcopyrite CuFeS2

Chalcopyrite,CuFeS2

Bornite, Cu5FeS4

Galena, PbSSphalerite, ZnSArsenopyrite,

FeAsS

Cinnabar, HgSStibnite, Sb2S3

Argentite, Ag2S

Gangue Minerals

QuartzTourmalineTopazMicas

QuartzCarbonatesBarite

QuartzChalcedonyOpalCalcite

Chalcopyrite (CuFeS2)

•Primary copper mineral in“porphyry-copper”deposits: sulfidesdesseminated in felsicintrusive rocks.

•The most widespreadcopper mineral.

•Usually meso-tohypothermal deposits.

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Galena (PbS)

•Primary ore mineral of Pb.

•Primarily found inmesothermal “MississippiValley Pb-Zn deposits”.

•Simple rocksalt structure.

•Forms large cubic crystals.

Sphalerite (ZnS)

•Primary ore mineral of Zn.

•Primarily found inmesothermal “MississippiValley Pb-Zn deposits”.

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Molybdenite (MoS2)

•Primary molybdenum ore.

•High-temperature deposits.Accessory in granites

Fundamental Questions

•How are metals such as Cu, Zn, Au and Pbconcentrated into ore deposits?

•What chemical signatures can we use to findore deposits?

•Are there vast resources at depth that wehaven’t yet discovered?

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Keq vs T

pK = -(ln K)/2.303 = ΔG0/(2.303RT)

= ΔH0/(2.303RT) - ΔS0/(2.303R)

€

pK(T ) = pK(298) +ΔH0

2.303R1T−

1298

If we assume ΔH0 and ΔS0 are constant with T, then

Since lnK = -ΔG0/RT we find,

Solubility of Sphalerite (ZnS)

ZnS + 2H+ = Zn+2 + H2S

Under acidic conditions, we can express the dissolutionof sphalerite as

For this reaction, pK = 4.44 and ΔH0 = 14.0 kJ/mol at298 K.

pK = pZn + pH2S - 2pH

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Solubility of Sphalerite (cont.)

€

pK(T ) = pK(298) +ΔH0

2.303R1T−

1298

= pZn+ pH2S − 2pH

€

pZn = 4.44 +14.0

2.303R1T−

1298

− pH2S + 2pH

Rearranging gives,

Solubility of Sphalerite (cont.)

Elevated temperaturesare not enough toaccount for thesolubilities of sulfideminerals needed tofrom ore-deposits.

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Nature of Ore-Forming Solutions

Fluid inclusions in mineral grains preserve samples ofhydrothermal solutions. Upon cooling, thehydrothermal brines separate into solid (usually NaCl,gas (CO2 + CH4) and aqueous phases.

The temperature at whichthe fluid was trapped canbe determined by heatingthe sample and measuringthe temperature at whichgas + liquid recombine.

Cl Complexation of Zn

Zn+2 + Cl- = ZnCl+

Zn+2 + 3Cl- = ZnCl3-

Zn+2 + 2Cl- = ZnCl2

Zn+2 + 4Cl- = ZnCl4-2

Zn(H2O)6 + nCl = ZnCln + 6H2O

pK = -0.2; ΔH = 43.3 kJ/mol

pK = -0.25; ΔH = 31.2 kJ/mol

pK = 0.02; ΔH = 22.6 kJ/mol

pK = -0.86; ΔH = 5.0 kJ/mol Complexation is drivenby the entropy increasewhen solvation watersare released.

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Cl Complexation of Zn

We can combine the reaction

ZnS + 2H+ = Zn+2 + H2S (pKZnS; ΔHZnS)

with each complexation reaction

Zn+2 + nCl- = ZnCln2-n (pKn; ΔHn)

to get the reactions

ZnS + 2H+ + nCl = ZnCln2-n + H2S

with pK = pKZnS + pKn and ΔH = ΔHZnS + ΔHn

Cl Complexation of Zn

€

pK(T ) = pK(298) +ΔH0

2.303R1T−

1298

= pZnCln2−n

+ pH2S − 2pH - npCl

To a close approximation, pCl = pCltot. Rearranginggives

€

pZnCln2−n

= pK(298) +ΔH0

2.303R1T−

1298

− pH2S + 2pH + npCl

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Solubility of Sphalerite: Cl complexation

Cl-complexation of Zngreatly enhances thesolubility of ZnS athigh temperature.

Caution: we assumedthat ΔH0 was constantwith T.

Entropy and Complexation

The complexation of metals at high temperature isdriven by the increased translation entropy resultingfrom the breakdown of the metal hydration sphere:

Zn(H2O)6 + Cl- = ZnCl(H2O)3+ + 3H2O

Zn(H2O)6 + 2Cl- = ZnCl20 + 6H2O

(Hydration numbers are derived from molecular dynamicssimulations.)

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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The Continuum Model of AqueousSolutions

Born (1920) theory of solvation free energy∆G:

Where:

R = “Born radius” of cation with charge q ε = dielectric constant of the solvent

Basis for HKF Equation of State used to predict stabilityconstants of complexes at high P,T.

€

ΔG =−q2e2

2R1−

1ε

Changes in Dielectric Constant of Waterwith P and T

•We expect decreasedsolvation of ions withincreasing T.

•This will favor metalcomplexation by Cl-.

•Pressure shouldenhance solvation.

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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HKF Equation of State (cont.)

€

Cp

0(P,T) = c1 +c2

(T −θ)2+ωTX + 2TY

∂ω∂T

P

−T1ε−1

∂2ω∂T 2

P

€

Vp

0(P,T) = a1 +a2

P +ψ+

a3

T −θ+

a4

P +ψ( ) T −θ( )−ωQ +

1ε−1

∂ω∂P

T

The heat capacity and volume of a species depend on Tand P as:

Where c1, c2, a1, a2, a3 and a4 are parameters for theparticular solute species…

HKF Equation of State (cont.)

ω is the Born coefficient of the ion,

And, finally, θ and ψ are parameters for the solvent.

€

Y =1ε2

∂ε∂T

P

, Q =1ε2

∂ε∂P

T

, X =1ε2

∂2ε∂T 2

P

−2ε

∂ε∂P

P

2

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Precipitation of Sulfides

Given the general reaction

ZnS + 2H+ +nCl = ZnCln2-n + H2S

ZnS will precipitate when H+ is consumed:

2H+ + CaCO3 (calcite) = CO2 + Ca+2 + H2O

3KAlSi3O8 (feldspar) + 2H+ = 6SiO2 + 2K+ + KAl3Si3O10(OH)2 (muscovite)

Volcanogenic Massive Sulfide Deposits

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Convergent Plate Boundaries

Porphyry Deposits

Phyllic: 3KAlSi3O8 + 2H+ = KAl3Si3O10(OH)2 + 6SiO2 + 2K+

Argillic: 2KAl3Si3O10(OH)2 + 2H+ +3H20  =  3Al2Si2O5(OH)4 + 2K+

Potassic

Ore zone: CuCl2 + FeCl2 +2H2S = CuFeS2 + 4H+ + 4 Cl-

Physical Chemistry of Minerals and Aqueous SolutionsDM Sherman, University of Bristol

2005/2006

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Summary

•Complexation of metals by Cl- (and possibly HS-) greatlyenhances the solubility of sulfides at high temperature

•Sulfide minerals are extremely insoluble.

•Hydrothermal solutions contain high concentrations of NaCl.

•Precipitation of sulfide minerals occurs either by cooling,boiling or by a drop in pH when fluids react with host rock(e.g., carbonates).