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