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Metal Solubility and Speciation
Metal Concentrations in Ore FluidsLA-ICPMS Fluid Inclusion Data
Skarns
Zn 5000 – 10,000 ppmPb 500 – 5,000 ppmAg 5 – 50 ppm
Ulrich et al. 1999 (Nature)Williams-Jones and Heinrich 2005 (Economic Geology)Klemm et al. 2008 (Mineralium Deposita)Samson et al., 2008 (Geology)
Porphyries
Cu 2000 – 10,000 ppmMo 500 – 1,500 ppmAu 80 – 800 ppb
Zinc content of crustal fluids
Zinc vs Lead in crustal fluids
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.
Water molecules may be considered to be a simple electrical dipoles
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
Properties of Water
Density Dielectric Constant
Ore Mineral Solubility as Simple Hydrated Ions
Complexation
Au SS
H
H
H
H
OH H
O
HH
O
H
H
O
H
H
O
H
H
O
2-Formation of soluble aqueous metal species, e.g. Au(HS)2
-
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 Rules and Aqueous-Metal ComplexesHard cations (large Z/r) prefer to bond with hard anions (ionic bonding) and soft cations (small Z/r) with soft anions (covalent bonding)
Hard Borderline SoftAcids
Fe2+,Mn2+,Cu2+
Zn2+>Pb2+,Sn2+,As3+>Sb3+=Bi3+
H+, Na+>K+ Mg2+>Ca2+>Sr2+>Ba2+
Al3+>Ga3+
Y3+,REE3+ (Lu>La)Mo6+>W6+>Mo4+>W4+
Mn4+,Fe3+,U6+>U4+
BasesF-,OH-,CO3
2->HCO3-
NH3,SO42->HSO4
-
Acetate, Oxalate
Cl-
Au+>Ag+>Cu+ Hg2+>Cd2+
Pt2+>Pd2+
HS->H2SCN-,I->Br-
Gold solubility
1.5 m NaClP = 1000 bar
0.5 m KClpH buffered by K-feldspar-muscovite
SS = 0.01 m
A fO2 buffered by hematite-magnetite
B fO2 and fS2 buffered by Magnetite-pyrrhotite-pyrite
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
β2
log
βn
16
14
12
10
β3 β4
100 200 3000
Temperature ºC
log
β11
3.5
3.0
2.5
Stability of Zinc Bisulphide Species
0 2 4 6 8 10
-5
-6
-7
-8
-9lo
g m
(Zn
) tota
l
150 ºCZ
n2+
Zn(HS)20
ZnS(HS)-
Zn(H
S) 3
-
pH
Zn2+ + nHS- = Zn(HS)n
2-n
Zn2+ + 2HS- = ZnS(HS)-
log βn = log aZn(HS)n2-n – log aZn2+ -nlog aHS-
log β11 = log aZnS(HS)- – log aZn2+ -2log aHS- -pH
Tagirov and Seward (2010)
Zn2+ + 2HS- = ZnS(HS)-
2 4 6 8 10 12
-3
-4
-6
-7
-8
-9
-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+
300 ºC; 500 bar; ΣS = 0.05 m
2 4 6 8 10 12
-3
-4
-5
-6
-7
-8
-9
pH
mNaCl = 2 (12 Wt%)
mNaCl = 0.2 (1 Wt%)
mNaCl = 0.01
log
m Z
n to
tal
Zn-HS speciesZn2+
150 ºC; 500 bar; ΣS = 0.05 m
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
Solubillity 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
Gold solubility
T = 250 oCP = 500 bar
1 m NaCl
SS = 0.001 m
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)
REE-fluoride solubility and REE Complexation
Association of HF at low pHand low solubility of REEPrecludes transport of REE as fluoride complexes.
Williams-Jones et al. (2012).
References
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.
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.
Williams-Jones, A.E., \midisov, A.A. and Samson, I,M, 2012. The hydrothermal mobility of the rare earth elements – a tale of “ceria” and “yttria”. Elements, 8, in press.