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Metal Solubility and Speciation

Metal Solubility and Speciation. Metal Concentrations in Ore Fluids. LA-ICPMS Fluid Inclusion Data. Porphyries. Skarns. Cu 2000 – 10,000 ppm Mo 500 – 1,500 ppm Au 80 – 800 ppb. Zn 5000 – 10,000 ppm Pb 500 – 5,000 ppm Ag 5 – 50 ppm. Ulrich et al. 1999 (Nature)

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Metal Solubility and Speciation

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  1. Metal Solubility and Speciation

  2. Metal Concentrations in Ore Fluids LA-ICPMS Fluid Inclusion Data Porphyries Skarns Cu 2000 – 10,000 ppm Mo 500 – 1,500 ppm Au 80 – 800 ppb Zn 5000 – 10,000 ppm Pb 500 – 5,000 ppm Ag 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)

  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. H Au S S H H H H H H H H H H H H H 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. 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) Hard Borderline Soft Acids H+, Na+>K+ Mg2+>Ca2+>Sr2+>Ba2+ Al3+>Ga3+ Y3+,REE3+ (Lu>La) Mo6+>W6+>Mo4+>W4+ Mn4+,Fe3+,U6+>U4+ Fe2+,Mn2+,Cu2+ Zn2+>Pb2+,Sn2+, As3+>Sb3+=Bi3+ Au+>Ag+>Cu+ Hg2+>Cd2+ Pt2+>Pd2+ Bases Cl- HS->H2S CN-,I->Br- F-,OH-,CO32->HCO3- NH3,SO42->HSO4- Acetate, Oxalate

  16. P = 1000 bar 1.5 m NaCl 0.5 m KCl pH buffered by K-feldspar-muscovite A fO2 buffered by hematite-magnetite SS = 0.01 m B fO2 and fS2 buffered by Magnetite-pyrrhotite-pyrite Gold solubility

  17. Stability of Zinc Chloride Species Zn2++ nCl- = ZnCln2-n log βn = log aZnCln2-n – log aZn2+ -nlog aCl- e.g., Zn2+ + 2Cl- = ZnCl20; β2 -4 ZnCl+ ZnCl42- 80 β2 10 ZnCl20 Zn2+ 60 β4 8 40 20 β1 ZnCl3- 150 ºC 6 log βn Percent Zn species 80 ZnCl+ ZnCl20 4 60 β3 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 βn = log aZn(HS)n2-n – log aZn2+ -nlog aHS- 16 Zn2++ 2HS- = ZnS(HS)- β3 β4 14 log β11 = log aZnS(HS)- – log aZn2+ -2log aHS- -pH β2 12 log βn -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 β11 Zn(HS)3- -9 2.5 0 2 4 6 8 10 0 100 200 300 pH Temperature ºC Tagirov and Seward (2010)

  19. Relative Importance of Chloride and Bisulphide complexation 300 ºC; 500 bar; ΣS = 0.05 m 150 ºC; 500 bar; ΣS = 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 mΣS SVP 300 Soluble 250 200 Temperature ºC Insoluble 150 10 ppm 100 ppm 100 1000 ppm 10000 ppm 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.

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