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Metal Solubility and Speciation
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Solvation and the Hydrogen Bond
Hydrogen bonds impart structure to water and ice.
Ice crystals
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Hydrogen bonds impart structure to water and ice.
Dielectric constant of water. Determined by creating an electrical field between two capacitor plates and measuring the voltage. The oriented dipoles create an internal field that opposes the external field. The dielectric constant is the ratio voltage in a vacuum over that in water.
The Dieletric Constant of Water
200 400 600
5030
70
90
2010
5
4
2
L+VCp
Kba
r
T OC
Dielectric Constant () of Water
Simple ion solvation (hydration) Complex ion solvation (hydration)
Metal Speciation in Water
Gold-Bisulphide Complexation
Au SS
H
H
H
H
OH H
O
HH
O
H
H
O
H
H
O
H
H
O
-Formation of soluble aqueous metal species, e.g. Au(HS)2
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Metal Speciation in Water Vapour
Rather than constituting widely dispersed molecules, water vapour comprises clusters of hydrogen-bonded water molecules.
Metal species, which are uncharged, dissolve in water vapour by attaching to clusters of water molecules via hydrogen-bonding.
Molecular dynamic simulation of solvation (hydration) in water vapour.
Potential Ligands for metal complexation
Ion-Pairing and Ligand availability
Dissociation constant of NaCl
Dissociation constant of HCl
Ionic (hard) Bonding
Transfer of electrons – electrostatic interaction
+_
Individual atoms with spherical electron clouds
Protons attract electron clouds and polarise each other
Covalent bond
Covalent (soft) bonding - polarisabilitySharing of electrons
Electronegativity and Chemical Bonding• Ionic bonding – maximise electronegativity difference• Covalent bonding – minimise eletronegativity difference
Pearson’s HSAB Principles and Aqueous Metal Complexes
Hard acids (large Z/r) bond with hard bases (ionic bonding) and soft acids (small Z/r) with soft bases (covalent bonding).
Hard Borderline Soft
Acids
Fe2+,Mn2+,Cu2+
Zn2+>Pb2+,Sn2+,As3+>Sb3+=Bi3+
H+, Na+>K+ Al3+>Ga3+
Y3+,REE3+ (Lu>La)Mo+6, W+6, U+6
Zr4+,Nb5+
Bases
F-,OH-,CO32- >HCO3
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SO42- >HSO4
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PO43-
Cl-
Au+>Ag+>Cu+ Hg2+>Cd2+
Pt2+>Pd2+
HS->H2SCN-,I->Br-
Pearson (1963)
Copper Speciation in Aqueous Liquid
1 m NaCl
1 m NaCl
Au/Ag Speciation in Aqueous Liquid
1 m NaCl
1 m NaCl
Zinc Speciation in Aqueous Liquid
10
8
6
4
2
100 200 300Temperature ºC
log
βn
β2
β4
β1
β3
Ruaya and Seward (1986)
Stability of Zinc Chloride Species
log βn = log aZnCln2-n – log aZn2+ -nlog aCl- Zn2+
+ nCl- = ZnCln2-n
e.g., Zn2+ + 2Cl- = ZnCl20; β2
-4
-4 -3 -2 -1 0 1
log Cl (mol/Kg)
80
604020
80
604020P
erce
nt Z
n sp
ecie
s Zn2+
ZnCl+
ZnCl20ZnCl+
ZnCl42-
ZnCl3-
ZnCl42-
ZnCl20
350 ºC
150 ºC
Molybdenum Speciation in Aqueous Liquid
Unlike most other metals, Mo, which occurs in hydrothermal fluids as Mo6+ is so hard that it reacts with water molecules to form covalently bonded, negatively charged molybdate species. The same is also true of W and U.
Minubayeva and Seward (2010 (2009)
Cu-Mo Zoning in Porphyry Systems
Mo
Cp
Aqueous fluid containing 2 m NaCl, 0.5 m KCl, 4000 ppm Cu and 1000 ppm Mo in equilibrium with K-feldspar, muscovite and quartz.
Gold speciation and transport
1.5 m NaClP = 1000 bar
0.5 m KClpH buffered by K-feldspar-muscovite
S = 0.01 m
A fO2 buffered by hematite-magnetite
B fO2 and fS2 buffered by Magnetite-pyrrhotite-pyrite
Williams-Jones et al. (2009)
2 4 6 8 10 12
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-4
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-2
-5
pH
mNaCl = 2 (12 Wt%)
mNaCl = 0.2 (1 Wt%)
mNaCl = 0.01
log
m Z
n to
tal
Zn-HS species
Zn-ClZn2+
2 4 6 8 10 12
-3
-4
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pH
mNaCl = 2 (12 Wt%)
mNaCl = 0.2 (1 Wt%)
mNaCl = 0.01
log
m Z
n to
tal
Zn-HS speciesZn2+
Zn-Cl
Tagirov and Seward (2010)
Relative Importance of Chloride and Bisulphide complexation
350
300
250
200
150
100
50
1 2 3 4 5 6 7 8 9 10
Tem
pera
ture
ºC
pH
10 ppm100 ppm1000 ppm10000 ppm
Solubility of Sphalerite as a Function of Temperature and pH
2m NaCl0.01 mΣSSVP
(Based on data of Ruaya and Seward 1986; Tagirov and Seward, 2010)
Soluble
Insoluble
A constraint on MVT Ore Formation
Although most researchers support a fluid mixing model for MVT deposits, some have proposed a single fluid model.
Our modelling shows that sphalerite will precipitate even in the presence of vanishingly small concentrations of H2S. Ore metals and reduced sulphur must be transported separately.
Metalliferous brine containing 15 wt.% NaCl and 1000 ppm Zn
Dashed lines (Haas et al., 1995), theoretical extrapolations from ambient temperature.
Solid lines (Migdisov et al., 2009) experimental determinations.
Note 1: REE fluoride complexes three orders of magnitude more stable than REE chloride complexesNote 2: Above 150 oC LREE complexes more stable than HREE complexes.
REE Fluoride and Chloride Complexes
Migdisov et al. (2009)
Modelling REE Mineral Solubility in a F-Bearing Brine
10 wt.% NaCl, 500 ppm F, 200 ppm Nd
The REE are transported dominantly as chloride complexes despite the greater stability of REE fluoride complexes, because HF is a weak acid and REE fluoride is relatively insoluble. Migdisov and Williams-Jones (2014)
Hydrothermal Fractionation of the REE
LREE are mobilised (as chloride complexes) relative to the HREE; REE are deposited as monazite.
Fluid contains 10 wt.%NaCl, 500 ppm F, and 50 ppm of each REE. Rock contains 100 ppm P.
Williams-Jones et al. (2012)
The Stability of REE-Sulphate Complexes
The stability of the REESO4
+ complexes is independent of atomic number.
The species REE(SO4)2
- are more stable stable than REESO4
+ .
Log
β1
Log
β2
250C
250C
Migdisov and Williams-Jones (2008)
Nd Speciation and solubility in a Cl-F-SO4-bearing Fluid
Log
m
pH
Nd Speciation and solubility in a fluid containing 10 wt.% NaCl, 500 ppm F, 2 wt.% Na2SO4 and 200 ppm Nd.
Ore-forming concentrations (> 1ppm Nd) are transported as NdCl2+
Log
m
pH
Nd Speciation and solubility in a Cl-F-SO4-bearing Fluid
At 400C NdCl2+ predominates to a pH of 3.5. Between this pH and a pH of 7.5 Nd(SO4)2
- predominates but is only able to transport ore-bearing concentrations (>1 ppm) at pH <5
Fluid/Rock Interaction as a Precipitation Mechanism for Sulphate-Complexed REE
As little as 230 mg per Kg of apatite is needed to precipitate all the Nd as Monazite-(Nd) (NdPO4).
Migdisov and Williams-Jones (2014)
Simplified Model for the Hydrothermal Transport and Deposition of REE
Mixing of magmatic and external fluids Fluid/rock interaction
REE Mineral Deposition
Mobilisation of REE as acidic REE-Cl complexes; weakly acidic REE-SO4 complexes at high T.
Chloride transport: Deposition of REE minerals, due to increasing pH, decreasing temperature and high activity of a depositional ligand.
Sulphate transport: Deposition of REE minerals due to interaction with a depositional ligand.
The effect of solvation make heavy metals volatile
HydrationReaction
Vapour transport - what did Krauskopf Ignore?
Cl
Au+
ClAu
Vapour Transport of CopperSolvation by clusters of water molecules at high water fugacity can can raise the solubility of copper as simple chlorides or sulphides to ore-forming concentrations.
Migdisov et al. (2014)
The Solubility of Chalcopyrite in Water Vapour
Increasing PH2O promotes hydration (and solubility) and increasing temperature inhibits hydration.
Solubility of Gold in HCl-H2O VapourDependence of Au solubility on fHCl of ~1 indicates formation of AuCl
Dependence of Au solubility on fH2O indicates hydration
Hurtig and Williams-Jones (2014)
HS Epithermal Au Ore Formation
Vapour-dominated hydrothermal plume rises from magma, transporting Au and depositing it as temperature drops below 400C.
Hurtig and Williams-Jones (2014)
References
Williams-Jones, A.E., and Migdisov, A., 2014, Experimental contraints on . The transport and deposition of metals in ore-forming hydrothermal systems. Society of Economic Geologists, Special Publication 18, pp 77-95.
Eugster, H.P., 1986, Minerals in hot water. American Mineralogist, v.71, 655-673.
Crerar, D., Wood, S.M., Brantley, S., and Bocarsly, A., 1985, Chemical controls on solubility of ore-forming minerals in hydrothermal solutions. Canadian Mineralogist, v. 23, p. 333-352
Seward, T.M., and Barnes, H.L., 1997, Metal transport by hydrothermal fluids in Geochemistry of Hydrothermal Ore Deposits H.L. Barnes (ed), p. 235-285. John Wiley and Sons Inc.