Hydrothermal Solutions
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Transcript of Hydrothermal Solutions
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.)
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pK(T ) = pK(298) +ΔH0
2.303R1T−
1298
= pZn+ pH2S − 2pH
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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
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pK(T ) = pK(298) +ΔH0
2.303R1T−
1298
= pZnCln2−n
+ pH2S − 2pH - npCl
To a close approximation, pCl = pCltot. Rearranginggives
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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.
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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.)
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Cp
0(P,T) = c1 +c2
(T −θ)2+ωTX + 2TY
∂ω∂T
P
−T1ε−1
∂2ω∂T 2
P
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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).