Metal Solubility and Speciation

1
Metal Solubility and Speciation
2
Metal Concentrations in Ore Fluids
LA-ICPMS Fluid Inclusion Data
Ulrich et al. 1999 (Nature) Williams-Jones and
Heinrich 2005 (Economic Geology) Klemm et al.
2008 (Mineralium Deposita) Samson et al., 2008
(Geology)
3
Zinc content of crustal fluids
4
Zinc vs Lead in crustal fluids
5
Solvation (Hydration)
The polar nature of the water molecule causes
separation of ionic species. The number of water
molecules surrounding an ion (hydration number )
depends on the ionic radius.
6
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.
Water molecules may be considered to be a simple
electrical dipoles
7
Properties of Water
Dielectric Constant
Density
8
Ore Mineral Solubility as Simple Hydrated Ions
9
Complexation
O
O
O
2-
Formation of soluble aqueous metal species, e.g.
Au(HS)2-
O
O
O
10
Potential Ligands for metal complexation
11
Ion-Pairing and Ligand availability
Dissociation constant of NaCl
Dissociation constant of HCl
12
Ionic (hard) Bonding
Transfer of electrons electrostatic interaction

_
13
Covalent (soft) bonding - polarisability
Sharing of electrons
Individual atoms with spherical electron clouds
Protons attract electron clouds and polarise each
other
Covalent bond
14
Electronegativity and Chemical Bonding

• Ionic bonding maximise electronegativity
  difference
• Covalent bonding minimise eletronegativity
  difference

15
Pearsons Rules and Aqueous-Metal Complexes
Hard cations (large Z/r) prefer to bond with hard
anions (ionic bonding) and soft cations (small
Z/r) with soft anions (covalent bonding)
16
Gold solubility
17
Stability of Zinc Chloride Species
-4
ZnCl
ZnCl42-
ß2
80
10
ZnCl20
Zn2
60
ß4
8
40
20
ß1
ZnCl3-
150 ºC
6
log ßn
Percent Zn species
80
ZnCl20
ZnCl
4
60
ß3
40
2
20
ZnCl42-
350 ºC
-4
-3
-2
-1
0
1
300
100
200
Temperature ºC
log Cl (mol/Kg)
Ruaya and Seward (1986)
18
Stability of Zinc Bisulphide Species
Zn2 nHS- Zn(HS)n2-n
log ßn log aZn(HS)n2-n log aZn2 -nlog aHS-
16
Zn2 2HS- ZnS(HS)-
ß3
ß4
14
log ß11 log aZnS(HS)- log aZn2 -2log aHS-
-pH
ß2
12
log ßn
-5
10
150 ºC
-6
Zn2
3.5
Zn(HS)20
Zn2 2HS- ZnS(HS)-
-7
log m(Zn)total
3.0
-8
ZnS(HS)-
log ß11
Zn(HS)3-
-9
2.5
0
2
4
6
8
10
100
200
300
0
pH
Temperature ºC
Tagirov and Seward (2010)
19
Relative Importance of Chloride and Bisulphide
complexation
300 ºC 500 bar SS 0.05 m
150 ºC 500 bar SS 0.05 m
-2
-3
mNaCl 2 (12 Wt)
mNaCl 2 (12 Wt)
-3
-4
mNaCl 0.2 (1 Wt)
mNaCl 0.2 (1 Wt)
-4
-5
mNaCl 0.01
-5
log m Zntotal
mNaCl 0.01
-6
log m Zntotal
-6
-7
-7
Zn2
Zn-Cl
-8
-8
Zn2
Zn-HS species
Zn-Cl
Zn-HS species
-9
-9
2
4
6
8
10
12
2
4
6
8
10
12
pH
pH
Tagirov and Seward (2010)
20
Solubillity of Sphalerite as a Function of
Temperature and pH
(Based on data of Ruaya and Seward 1986 Tagirov
and Seward, 2010)
350
2m NaCl 0.01 mSS SVP
300
Soluble
250
200
Temperature ºC
Insoluble
150
100
50
1
2
3
4
5
6
7
8
9
10
pH
21
Gold solubility
T 250 oC
P 500 bar
1 m NaCl
SS 0.001 m
22
REE Complexation
REE forms very stable fluoride complexes, and
less stable chloride complexes The LREE are much
more mobile than the LREE
Migdisov et al. (2009)
23
REE-fluoride solubility and REE Complexation
Association of HF at low pH and low solubility of
REE Precludes transport of REE as fluoride
complexes.
Williams-Jones et al. (2012).
24
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, 655-673.
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 Heinrich C.A., 2005,
Vapor transport of metals and the formation of
magmatic-hydrothermal ore deposits. Economic
Geology 100 1287-1312.