Gold solubility and speciation in hydrothermal solutions: Experimental study of the stability of hydrosulphide complex of gold (AuHS degrees) at 350 to 450 degrees C and 500 bars

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 degrees C at a constant pressure of 500 bars. Two buffer assemblages we re used to constrain fO(2),fS(2), and consequently fH(2) and aH(2)S: respec tively, pyrite-pyrrhotite-magnetite (Py-Po-Mt) and pyrite-magnetite-hematit e (Py-Mt-Hm). The measured solubility of gold at equi librium with Py-Po-Mt and Qtz-KF-Mus is 52 +/- 8 ppb at 350 degrees C, 134 +/- 17 ppb at 400 deg rees C and 496 +/- 37 ppb at 450 degrees C. With Py-Mt-Hm and Qtz-KF-Mus th e solubility of gold is increased to 198 +/- 9 ppb at 400 degrees C and 692 +/- 10 ppb at 450 degrees C. These results are consistent with the aqueous complex AuHS degrees being the dominant gold-bearing species. The equilibr ium constants (log K-R10) for the reaction: Au-(s) + H2S(aq) = AuHS0 + 1/2H(2(g)) R10 have been determined at 350, 400, and 450 degrees C and are, respectively, -5.20 +/- 0.25, -5.30 +/- 0.15, and -5.40 +/- 0.15. These values are simila r 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 t he possible causes of the discrepancies are discussed. The equilibrium cons tant for AuHS degrees shows that this species plays an important role in th e deposition of gold in natural environments. Cooling, H2S loss, pH change, and oxidation seem to be effective mechanisms for gold precipitation, depe nding on the local ore forming conditions. Copyright (C) 1998 Elsevier Scie nce Ltd.