Metal Solubility and Speciation
Solvation and the Hydrogen Bond
-
-
+
+
+
+
-
+
+
-
+
-
+
-
+
-
Ice crystals
+
+
+
-
+
+
+
Hydrogen bonds impart structure
to water and ice.
The Dieletric Constant of Water
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.
Dielectric Constant () of Water
4
90
Kbar
70
50
30
20
2
10
5
200
400
T OC
600
Metal Speciation in Water
Simple ion solvation (hydration)
Complex ion solvation (hydration)
Gold-Bisulphide Complexation
O
H
-
H
Formation of soluble
aqueous metal species,
e.g. Au(HS)2-
H
S
H
H
O
Au
H
S
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
+
_
Covalent (soft) bonding - polarisability
Sharing of electrons
Individual atoms with
spherical electron clouds
Protons attract electron clouds
and polarise each other
Covalent bond
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
H+, Na+>K+
Al3+>Ga3+
Y3+,REE3+ (Lu>La)
Mo+6, W+6, U+6
Zr4+,Nb5+
Fe2+,Mn2+,Cu2+
Zn2+>Pb2+,Sn2+,
As3+>Sb3+=Bi3+
Au+>Ag+>Cu+
Hg2+>Cd2+
Pt2+>Pd2+
Bases
F-,OH-,CO32- >HCO3SO42- >HSO4PO43-
Cl-
HS->H2S
CN-,I->BrPearson (1963)
Copper Speciation in Aqueous Liquid
1 m NaCl
1 m NaCl
Au/Ag Speciation in Aqueous Liquid
Zinc Speciation in Aqueous Liquid
1 m NaCl
1 m NaCl
Stability of Zinc Chloride Species
Zn2+ + nCl- = ZnCln2-n
log βn = log aZnCln2-n – log aZn2+ -nlog aCle.g., Zn2+ + 2Cl- = ZnCl20; β2
β4
log βn
8
β1
6
4
β3
2
100
200
300
Temperature ºC
Percent Zn species
β2
10
80
60
40
20
80
60
40
20
ZnCl+
Zn2+
ZnCl42-
ZnCl20
ZnCl3-
150 ºC
ZnCl+
-4
ZnCl20
ZnCl42-
350 ºC
-4
-3
-2
-1
0
1
log Cl (mol/Kg)
Ruaya and Seward (1986)
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
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.
Cp
Mo
Gold speciation and transport
P = 1000 bar
1.5 m NaCl
0.5 m KCl
pH buffered by Kfeldspar-muscovite
A fO2 buffered by
hematitemagnetite
SS = 0.01 m
B fO2 and fS2
buffered by
Magnetitepyrrhotite-pyrite
Williams-Jones et al. (2009)
Relative Importance of Chloride and
Bisulphide complexation
-2
-3
mNaCl = 2 (12 Wt%)
-5
mNaCl = 0.01
-7
-8
Zn2+
Zn-Cl
mNaCl = 0.2 (1 Wt%)
-4
log m Zntotal
log m Zntotal
mNaCl = 0.2 (1 Wt%)
-6
mNaCl = 2 (12 Wt%)
-3
-4
mNaCl = 0.01
-5
-6
-7
Zn2+
Zn-Cl
-8
Zn-HS species
-9
Zn-HS species
-9
2
4
6
8
pH
10
12
2
4
6
8
10
12
pH
Tagirov and Seward (2010)
Solubility of Sphalerite as a Function of
Temperature and pH
(Based on data of Ruaya and Seward 1986; Tagirov and Seward, 2010)
Temperature ºC
350
2m NaCl
0.01 mΣS
SVP
300
250
Soluble
200
Insoluble
150
10 ppm
100 ppm
1000 ppm
10000 ppm
100
50
1
2
3
4
5
pH
6
7
8
9
10
A constraint on MVT Ore Formation
Although most researchers support a fluid mixing model for MVT
deposits, some have proposed a single fluid model.
Metalliferous
brine
containing 15
wt.% NaCl and
1000 ppm Zn
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.
REE Fluoride and Chloride Complexes
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 complexes
Note 2: Above 150 oC
LREE complexes
more stable than
HREE complexes.
Migdisov et al. (2009)
Modelling REE Mineral Solubility in a FBearing 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 the
REESO4+ complexes is
independent of atomic
number.
Log β1
The Stability of REE-Sulphate
Complexes
The species
REE(SO4)2- are more
stable stable than
REESO4+ .
Log β2
250C
250C
Migdisov and Williams-Jones (2008)
Log m
Nd Speciation and solubility in a
Cl-F-SO4-bearing Fluid
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
Nd Speciation and solubility in a
Cl-F-SO4-bearing Fluid
pH
At 400C 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.
Vapour transport - what did
Krauskopf Ignore?
The effect of solvation make heavy metals volatile
Hydration
Reaction
Cl
+
Au
Au
Cl
Vapour Transport of Copper
Solvation 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 Vapour
Dependence 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 400C.
Hurtig and Williams-Jones (2014)
References
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
Eugster, H.P., 1986, Minerals in hot water. American Mineralogist, v.71, 655673.
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.
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.