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pH and oxidation state is shown on Figure 3. Also shown on Figure 3 are the stability

fields for Fe-bearing phases hematite, pyrite, magnetite and pyrrhotite for a total sulfur

activity of 0.01. Illite and adularia are common alteration and gangue minerals in

epithermal systems, and the illite-adularia equilibrium boundary is shown for both

equilibrium with amorphous silica and quartz at 250°C, assuming K+ concentration of 5 x

10-3 m and Mg2+ concentration of 4 x 10-5 m. Illite-adularia equilibria were calculated

using data from Helgeson (1969) combined with the data from Gunnarsson and

Arnórsson (2000) for quartz and amorphous silica equilibria. Note that the pH of a fluid

in equilibrium with illite and adularia and precipitating quartz will decrease by greater

than one pH unit if the fluid suddenly begins to precipitate amorphous silica in response

to boiling. This results in a decrease in the gold solubility of about 1-2 orders of

magnitude and can cause gold to precipitate. Also shown o Figure 3 are the H2S – HS-

and HSO-4 – SO2-4 equilibrium boundaries. In most epithermal deposits, pyrite or an iron

oxide phase and adularia are common gangue minerals (Sillitoe and Hedenquist, 2003).

Gold solubilities shown on Figure 3 are consistent with Au concentrations in deep

geothermal waters in the Taupo Volcanic Zone and at Lihir Island, Papua New Guinea

(Simmons, and Brown, 2008).

According to Figure 3, the maximum gold solubility occurs near the intersection

of the hematite and pyrite fields and decreases in all directions away from this maximum.

Assuming a fluid is saturated in gold at this maximum, any process that causes the fluid

composition to move away from that point results in a decrease in solubility and

deposition of gold. Thus, gold can be deposited if the fluid pH increases or decreases, or

if the oxygen fugacity increases or decreases (Fig. 3). Assuming that gold is transported


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