Metal Solubility and Speciation

++ -- ++ ++ -- ++ -- -- -- -- -- -- -- -- ++ -- ++ ++ ++ ++ ++ ++ -- ++ ++ ++ ++ Solvation and the Hydrogen Bond Hydrogen bonds impart structure to water and ice. Ice crystals ++ -- ++ ++ -- ++ -- -- -- -- -- -- -- -- ++ -- ++ ++ ++ ++ ++ ++ -- ++ ++ ++ ++ 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

L+V Cp Kbar T O C Dielectric Constant (  ) of Water

Simple ion solvation (hydration)Complex ion solvation (hydration) Metal Speciation in Water

Gold-Bisulphide Complexation Au S S H H H H O H H O H H O H H O H H O H H O - Formation of soluble aqueous metal species, e.g. Au(HS) 2 -

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 - polarisability Sharing 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 BorderlineSoft Acids Fe 2+,Mn 2+,Cu 2+ Zn 2+ >Pb 2+,Sn 2+, As 3+ >Sb 3+ =Bi 3+ H +, Na + >K + Al 3+ >Ga 3+ Y 3+,REE 3+ (Lu>La) Mo +6, W +6, U +6 Zr 4+,Nb 5+ Bases F -,OH -,CO 3 2- >HCO 3 - SO 4 2- >HSO 4 - PO 4 3- Cl- Au + >Ag + >Cu + Hg 2+ >Cd 2+ Pt 2+ >Pd 2+ HS - >H 2 S CN -,I - >Br - Pearson (1963)

Copper Speciation in Aqueous Liquid 1 m NaCl

Au/Ag Speciation in Aqueous Liquid

1 m NaCl Zinc Speciation in Aqueous Liquid

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

Molybdenum Speciation in Aqueous Liquid Unlike most other metals, Mo, which occurs in hydrothermal fluids as Mo 6+ 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 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 Williams-Jones et al. (2009)

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 Solubility 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

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 H 2 S. 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 complexes Note 2: Above 150 o C 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 REESO 4 + complexes is independent of atomic number. The species REE(SO4) 2 - are more stable stable than REESO 4 +. Log β 1 Log β  C Migdisov and Williams-Jones (2008)

Nd Speciation and solubility in a Cl-F-SO 4 -bearing Fluid Log m pH Nd Speciation and solubility in a fluid containing 10 wt.% NaCl, 500 ppm F, 2 wt.% Na 2 SO 4 and 200 ppm Nd. Ore-forming concentrations (> 1ppm Nd) are transported as NdCl 2 +

Log m pH Nd Speciation and solubility in a Cl-F-SO 4 -bearing Fluid At 400  C NdCl 2 + predominates to a pH of 3.5. Between this pH and a pH of 7.5 Nd(SO 4 ) 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) (NdPO 4 ). 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-SO 4 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 Hydration Reaction Vapour transport - what did Krauskopf Ignore? Cl Au + Cl Au

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 PH 2 O promotes hydration (and solubility) and increasing temperature inhibits hydration.

Solubility of Gold in HCl-H 2 O Vapour Dependence of Au solubility on fHCl of ~1 indicates formation of AuCl Dependence of Au solubility on fH 2 O 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 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 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.