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Epithermal Deposits
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Epithermal SystemsLow and high sulphidation deposits
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Submarine Epithermal Systems
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The significance of Epithermal Deposits as a Gold Resource
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Distribution of Epithermal Deposits
High Sulphidation
Low Sulphidation
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Surface expression of a low sulphidation epithermal deposit
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Low Sulphidation DepositsOre Styles and Alteration Assemblages
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Low Sulphidation DepositsFluid Inclusion Temperatures and Salinities
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Low Sulphidation DepositsOxygen and Hydrogen Isotopic Data
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Low Sulphidation DepositsTemperature-pH Conditions
500
400
300
2001 2 3 4 5 6
oC
Log mK+/mH+
Muscovite K-feldspar
Kaolinite/dickite
Andalusite
3 KAlSi3O8 + 2 H+ = KAl3Si3O10(OH)2 + 6 SiO2 + 2 K+
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The Low Sulphidation Epithermal –Geothermal Link
Champagne Pool, New Zealand
Up to 90 g/t Au
Old Faithful, Yellowstone
Wairakei Geothermal Power Plant, New Zealand
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The Low Sulphidation Epithermal –Geothermal Link
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Geothermal Well Scalingsfrom Cerro Prieto, Mexico
Electrum
ChalcopyriteSphalerite
Clark, J.R. & Williams-Jones, A.E., (1990) Analogues of epithermal gold-silver deposition in geothermal well scales: Nature, v. 346, no. 6285, pp 644-645.
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Controls on the Solubility of Gold
Au(HS)2- +H+ + 0.5 H2O
= Au + 2H2S +0.25O2
AuCl2- + 0.5H2O= Au + 2Cl- + H+ +0.25O2
Williams-Jones et al. 2009
Au(HS)o + 0.5 H2O= Au + H2S +0.25O2
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A model for the formation of low sulphidation epithermal deposits
1) Magmatic vapour condenses in meteoric water2) Gold transported as Au (HS)2
-
3) Water rises and boils, releasing H2S and destabilizing Au(HS)2-4) Gold deposits as the native metal
Au(HS)2- + H+ + 0.5 H2O
= Au + 0.25O2 + 2H2S
Removed by boiling
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Epithermal SystemsHigh sulphidation deposits
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High Sulphidation DepositsOre Style and Alteration Assemblages
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Acid-Sulphate Alteration
Vuggy silica Advanced argillic alteration
All components of the rock leached leaving behind vuggy silica (pH < 1)
Alunite (KAl3(SO4)2(OH)6Kaolinite (Al2Si2O5(OH)4Quartz and Pyrite
Pyrite
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Conditions of Acid-Sulphate Alteration
King et al., 2014
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Chouinard et al., 2005
The high sulphidation Pascua epithermal deposit, Chile
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Mineralization at Pascua
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High Sulphidation DepositsOxygen and Hydrogen Isotopic Data
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High Sulphidation DepositsFluid Inclusion Temperatures and Salinities
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A Model for the Formation of High Sulphidation Deposits
Cooke and Simmons, 2000
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Controls on the Solubility of Gold
Au(HS)2- +H+ + 0.5 H2O
= Au + 2H2S +0.25O2
AuCl2- + 0.5H2O= Au + 2Cl- + H+ +0.25O2
Williams-Jones et al. 2009
Au(HS)o + 0.5 H2O= Au + H2S +0.25O2
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Sangihe
Lessons from Indonesia
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The Sangihe Au-Ag Deposits
Py I Au 1.1 ppmAg 33 ppm
Py II Au 1 ppmAg 81 ppm
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Copper map for Py II at Sangihe
Gold map forpyrite at Pascua
Cu (green) As (blue) maps forpyrite at Pascua
Metal zoning in pyrite
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The Lycurgus Cup – dichroic glass and nanogold
A possible explanation for “invisible gold” in pyrite – electrostatic attractionof negatively charged nanogold particles to the surfaces of positively chargedpyrite Williams-Jones et al. 2009
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The Sangihe Model
King et al.(2014)
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Kawah Ijen - High Sulphidation Epithermal Deposit in the Making?
Dacite Dome Alunite/pyrite
Acid lake pH 0.5
Mining sulphur
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4H2O (gas) + 4SO2(gas) = 2S (solid) + 2H2SO4 (gas)
H2SO4(aq) = 2H+ + SO42-
Sulphur condensationand acidity creation
600 oCpH -O.6
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Sampling the gases
Giggenbach bottle
Gas condenser
New dome
Alunite/pyrtite Au?
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Vapour-induced acid-sulphate Alteration
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Leached andesite pillow containing > 85 wt.% SiO2 – residual silica
Acid-Sulphate Alteration
Pyroclastic rocks altered to alunite (KAl3(SO4)2(OH)6 and pyrite
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Acid Sulphate Alteration at Kawah Ijen
Alunite-pyrite alteration Alunite-pyrite vein
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Distribution of Alteration at Kawah Ijen
Scher et al. (2013)
Vuggy silicaAlunite-pyriteKaolinitePyrite
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Gold Silver mineralisation at Kawah Ijen
Sangihe
Sangihe
Kawah Ijen
Kawah Ijen
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Solubility of Silver in HCl-H2O VapourSilver solubility increases with hydration
Migdisov and Williams-Jones (2013)
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Water Clusters Hydrating a Metal Species in the Gas Phase
Williams-Jones and Migdisov (2014)
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The effects of complexation and particularly solvation by H2O clusters make heavy metals volatile
HydrationReaction
Think about clowns and balloons
Cl
Au+ ClAu
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Extracting Thermodynamic Data
Log K = -ΔG/RTMigdisov and Williams-Jones (2013)
The linear relationship between ΔG and reciprocal temperature enables extrapolation to high temperature
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Solubility of Silver in HCl-H2O VapourHydration increases with increasing H2O pressure or density but decreases with increasing temperatureSolubility increases with increasing temperature but reaches a maximum because of the effect of decreasing hydration
Migdisov and Williams-Jones (2013)
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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)
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References
Williams-Jones, A.E. and Heinrich, C.H., 2005, Vapor transport of metals and the formation of magmatic-hydrothermal ore deposits: Econ. Geol., 100, p.1287-1312.
King, J., Williams-Jones, A.E., van Hinsberg, V., and Williams-Jones, G. High sulfidation epithermal pyriote-hosted Au (Ag-Cu) ore formation by condensed magmatic vapors on Sangihe Island, Indonesia: Economic Geology, v. 109, p. 1705-1733.
Cooke, D.R, Simmons, S.F., 2000. Characteristics and genesis of epithermal gold deposits. In: Hagemann, S.G., Brown, P.E. (Eds.), Gold in 2000, Reviews in Econ. Geol. vol. 13. Society of Economic Geology, Boulder, CO, pp. 221–244.
Clark, J.R. and Williams-Jones, A.E., (1990) Analogues of epithermal gold-silver deposition in geothermal well scales: Nature, v. 346, no. 6285, pp 644-645.
Chouinard, A., Williams-Jones, A.E., Leonardson, R.W., Hodgson, C.J., Silva,P., Téllez, C, Vega, J., and Rojas, F., 2005a, Geology and genesis of the multistage high-sulfidation epithermal Pascua Au-Ag-Cu deposit, Chile and Argentina: Econ. Geol., v. 100, p. 463–490.
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Hurting, N., and Williams-Jones, A.E., 2014. An experimental study of the transport of gold through hydration of AuCl in aqueous vapor and vapor-like fluids. Geochimica et Cosmochimica Acta, 127, 305-325.
Migdisov, A.A. and Williams-Jones, A.E., 2013. A predictive model for the transport of silver chlioride by aqueous vapor in ore-forming magmatic-hydrothermal systems. Geochmica et Cosmochimica Acta, 104, 123-135.
Scher, S., Williams-Jones, A.E., and Williams-Jones, G., 2013. Fumarolic activity, acid sulfate alteration and high sulfidation epithermal precious metal mineralization in the crter of Kawah Ijen volcano (Java, Indonesia). Economic Geology, 108, 1099-1118.
Williams-Jones, A.E., and Migdisov, A.A., 2014. Experimental constraints on the transport and deposition of metals in ore-forming hydrothermal systems. Society of Economic Geologists, Specia;l Publication 18, 77-95.