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as a bisulfide complex, boiling of a solution results in consumption of hydrogen ion (pH

increases) and loss of the complexing agent for gold according to equation (1)

Au(HS)2- + 0.5 H2 + H+ = Au (s) + H2S (v)

(1)

Boiling also results in loss of CO2 from the solution which in turn leads to a pH

increase according to equation (2)

HCO3- + H+ = H2O + CO2 (v)

(2)

Pyrite precipitation also promotes gold deposition by removing the complexing

agent (sulfur) responsible for transporting gold (Henley and Brown, 1985) according to

equation (3)

Fe2+

  • +

    2Au(HS)2- = 2Au (s) + FeS2 (s) + 2H+ + 2HS-

(3)

Most epithermal ore-forming fluids are probably not saturated in Au initially, but

rather achieve saturation and deposit gold as a result of changes in the chemical and

physical environment at the site of deposition. However, as noted above, boiling, pyrite

deposition and a change from quartz to amorphous silica deposition all require changes in

the pH and/or sulfur activity of the fluid, which can lead to gold deposition.

The discussion above applies only to gold solubility and deposition. The

mechanisms by which silver is dissolved and transported in epithermal systems, on the

8

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