Metal Solubility and Speciation

Metal Concentrations in Ore Fluids LA-ICPMS Fluid Inclusion Data Skarns Zn 5000 – 10,000 ppm Pb 500 – 5,000 ppm Ag 5 – 50 ppm Ulrich et al (Nature) Williams-Jones and Heinrich 2005 (Economic Geology) Klemm et al (Mineralium Deposita) Samson et al., 2008 (Geology) Porphyries Cu 2000 – 10,000 ppm Mo 500 – 1,500 ppm Au 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 S S H H H H O H H O H H 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 - polarisability Sharing of electrons

Electronegativity and Chemical Bonding Ionic bonding – maximise electronegativity difference Covalent bonding – minimise eletronegativity difference

Pearson’s 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) HardBorderlineSoft Acids Fe 2+,Mn 2+,Cu 2+ Zn 2+ >Pb 2+,Sn 2+, As 3+ >Sb 3+ =Bi 3+ H +, Na + >K + Mg 2+ >Ca 2+ >Sr 2+ >Ba 2+ Al 3+ >Ga 3+ Y 3+,REE 3+ (Lu>La) Mo 6+ >W 6+ >Mo 4+ >W 4+ Mn 4+,Fe 3+,U 6+ >U 4+ Bases F -,OH -,CO 3 2- >HCO 3 - NH 3,SO 4 2- >HSO 4 - Acetate, Oxalate Cl- Au + >Ag + >Cu + Hg 2+ >Cd 2+ Pt 2+ >Pd 2+ HS - >H 2 S CN -,I - >Br -

Gold solubility 1.5 m NaCl P = 1000 bar 0.5 m KCl pH buffered by K- feldspar-muscovite  S = 0.01 m A fO 2 buffered by hematite- magnetite B fO 2 and fS 2 buffered by Magnetite- pyrrhotite-pyrite

Temperature ºC log β n β2β2 β4β4 β1β1 β3β3 Ruaya and Seward (1986 ) Stability of Zinc Chloride Species log β n = log aZnCl n 2-n – log aZn 2+ -nlog aCl - Zn 2+ + nCl - = ZnCl n 2-n e.g., Zn Cl - = ZnCl 2 0 ; β log C l (mol/Kg) Percent Zn species Zn 2+ ZnCl + ZnCl 2 0 ZnCl + ZnCl 4 2- ZnCl 3 - ZnCl 4 2- ZnCl ºC 150 ºC

β2β2 log β n β3β3 β4β Temperature ºC log β Stability of Zinc Bisulphide Species log m(Zn) total 150 ºC Zn 2+ Zn(HS) 2 0 ZnS(HS) - Zn(HS) 3 - pH Zn 2+ + nHS - = Zn(HS) n 2-n Zn HS - = ZnS(HS) - log β n = log aZn(HS) n 2-n – log aZn 2+ -nlog aHS - log β 11 = log aZnS(HS) - – log aZn 2+ -2log aHS - -pH Tagirov and Seward (2010) Zn HS - = ZnS(HS) -

pH mNaCl = 2 (12 Wt%) mNaCl = 0.2 (1 Wt%) mNaCl = 0.01 log m Zn total Zn-HS species Zn-Cl Zn pH mNaCl = 2 (12 Wt%) mNaCl = 0.2 (1 Wt%) mNaCl = 0.01 log m Zn total Zn-HS species Zn 2+ Zn-Cl Tagirov and Seward (2010) Relative Importance of Chloride and Bisulphide complexation

Temperature ºC pH 10 ppm 100 ppm 1000 ppm ppm Solubillity of Sphalerite as a Function of Temperature and pH 2m NaCl 0.01 mΣS SVP (Based on data of Ruaya and Seward 1986; Tagirov and Seward, 2010) Soluble Insoluble

Gold solubility T = 250 o C P = 500 bar 1 m NaCl  S = 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 pH and low solubility of REE Precludes 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: Eugster, H.P., 1986, Minerals in hot water. American Mineralogist, v.71, 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 Seward, T.M., and Barnes, H.L., 1997, Metal transport by hydrothermal fluids in Geochemistry of Hydrothermal Ore Deposits H.L. Barnes (ed), p John Wiley and Sons Inc. Williams-Jones, A.E., \midisov, A.A. and Samson, I,M, The hydrothermal mobility of the rare earth elements – a tale of “ceria” and “yttria”. Elements, 8, in press.