Computer Science
Scientific paper
Aug 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2111h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 8, pp.2111-2126
Computer Science
14
Scientific paper
A total of 108 silica capsule experiments were performed in order to determine the solubility of gold in aqueous solutions containing both NaCl and H 2 S at temperatures of 250, 300, and 350°C, at pressures dictated by vapor/liquid coexistence. The starting materials in the capsules were H 2 O + S ° + gold wire + NaCl (0 to 4 m ) ± Na 2 SO 4 . The pH, a H 2 S ( aq ), aH 2 ( aq ), f H 2 (g) , f O 2 (g) , and f S 2 (g) values of the solutions were controlled by the sulfur hydrolysis reaction (4 S ° + 4 H 2 O (1) = 3 H 2 S ( aq ) + HSO - 4 + H + ) and the sulfide/sulfate reaction. The quenched run products were analyzed for Au (0.1 to 66 ppm), H 2 S, SO 2- 4 , and pH. The calculated solution compositions at 250-350°C fall in the following ranges: pH = 1.9 to 5.0, log aH 2 S ( aq ) = -2.0 to -0.7, log a HSO 4 - = -3.8 to -1.4 , and log a H 2 ( aq ) = -7-0 to -4.7. The results of our experiments indicate that gold solubility is independent of the activity of Cl - and H + in the solutions, indicating that chloride complexes are not important. The gold solubility, however, increases with increasing H 2 S(aq) activity, indicating that gold dissolved largely as a bisulfide complex according to the reaction: . The equilibrium constant determined from our experimental data for the above reaction is constant over a temperature range of 250 to 350°C at log K = -5.1 ± 0.3. The above equilibrium constant, together with that for a reaction involving Au(HS) - 2 obtained by Shenberger and Barnes (1989), determines the dissociation constant of HAu (HS) 0 2 : HAu ( HS ) 0 2 = H + + Au ( HS ) - 2 . The log K value becomes -5.3 ± 0.5 at 250°C, -5.6 ± 0.6 at 300°C, and -6.2 ± 0.6 at 350°C. Therefore, HAu(HS) 0 2 is the dominant gold-sulfide species at pH below about 5.5, while Au(HS) - 2 becomes dominant at higher pH conditions. The results from our study suggest that the solubility of gold in ore-forming fluids in equilibrium with pyrite and/or pyrrhotite at 250-350°C is typically between 0.1 ppb to 1 ppm Au, transported mostly as bisulfide complexes; gold-chloride complexes do not become important unless the fluid is H 2 S-poor (e.g., <10 -4 m H 2 S(aq) at 250°C), chloride-rich (>0.5 m 2Cl), and of low pH (<4.5). Precipitation of gold from ore-forming solutions may occur by increasing a H 2 ( aq ), such as by reactions with organic matter or ferrous-bearing minerals, or by decreasing a H 2 S ( aq ), such as by precipitation of sulfide minerals or by mixing of H 2 S-poor fluids. Simple cooling or heating of fluids without changing the H 2 (aq) and H 2 S(aq) contents is not an effective mechanism of gold precipitation when gold is transported largely as a bisulfide complex. Effects of oxidation or of boiling on gold precipitation, however, cannot be easily evaluated from the available thermodynamic data alone.
Hayashi Ken-Ichiro
Ohmoto Hiroshi
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