Computer Science
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.2931g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 62, Issue 17, pp.2931-2947
Computer Science
8
Scientific paper
The solubility of gold was measured in aqueous KCl (0.5 m ) solutions under oxygen, sulfur, and slightly acidic pH buffered conditions between 350 and 450°C at a constant pressure of 500 bars. Two buffer assemblages were used to constrain f O 2 , f S 2 , and consequently f H 2 and a H 2 S: respectively, pyrite-pyrrhotite-magnetite (Py-Po-Mt) and pyrite-magnetite-hematite (Py-Mt-Hm). The measured solubility of gold at equilibrium with Py-Po-Mt and Qtz-KF-Mus is 52 ± 8 ppb at 350°C, 134 ± 17 ppb at 400°C and 496 ± 37 ppb at 450°C. With Py-Mt-Hm and Qtz-KF-Mus the solubility of gold is increased to 198 ± 9 ppb at 400°C and 692 ± 10 ppb at 450°C. These results are consistent with the aqueous complex AuHS° being the dominant gold-bearing species. The equilibrium constants (log K R10 ) for the reaction: Au ( s ) + H 2 S ( aq ) = AuHS 0 +1/2 H 2( g ) R10 have been determined at 350, 400, and 450°C and are, respectively, -5.20 ± 0.25, -5.30 ± 0.15, and -5.40 ± 0.15. These values are similar to those suggested by Zotov (written pers. commun.) and those obtained by recalculating the experimental data of Hayashi and Ohmoto (1991) . They are significantly higher than those derived by Benning and Seward (1996) and the possible causes of the discrepancies are discussed. The equilibrium constant for AuHS° shows that this species plays an important role in the deposition of gold in natural environments. Cooling, H 2 S loss, pH change, and oxidation seem to be effective mechanisms for gold precipitation, depending on the local ore forming conditions.
Gibert Fabien
Pascal Marie-Lola
Pichavant Michel
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