EXPERIMENTAL INVESTIGATION OF GOLD SOLUBILITY IN HIGH-TEMPERATURE VAPORS: THE KEY ROLE OF ALKALI METALS IN EFFICIENT GOLD TRANSFER

We studied the solubility of Au in aqueous vapor phases at 1000 ^oC and 150 MPa. The experiments have been conducted in rapid-quench TZM pressure vessels in a mixed Ar-H₂ pressure medium at logfO₂=NNO-0.4±0.3. Synthetic fluid inclusions were trapped in pre-fractured and in situ fractured quartz chips placed in Au capsules along with solutions of various compositions (H₂O, HCl-H₂O, NaCl-H₂O, HCl-NaCl-H₂O, S-H₂O, S-NaCl-H₂O, S-KCl-H₂O), and later analyzed by LA-ICPMS.

The solubility of gold in pure H₂O is below the limits of detection of 70 ng/g, showing the absence of hydroxy-complexes. The solubility of Au in pure aqueous HCl solution is low and linearly correlates with the HCl concentration (1.4 to 3.8 µg/g in 0.25 to 0.75 m HCl). The solubility in pure NaCl solution is somewhat higher (6.5±2.5 µg/g in 0.75 m NaCl). Remarkably, mixed NaCl-HCl solutions can dissolve significantly higher concentrations of Au compared to the pure end members (24±6 µg/g at 0.75 m total Cl and molar NaCl/HCl=1). This can be explained by association of Au with not only Cl but also Na following the dissolution reaction below:

Au + NaCl + HCl = NaAuCl₂ + ½ H₂

In H₂S-bearing, Cl-free vapors the solubility of Au is significantly higher compared to chloride-bearing, S-free solutions. The gold concentration shows an increase from 3.4±1.2 to 91±22 µg/g as the concentration of S increases from 1.0 wt% to 10.2 wt%. Addition of alkali-chlorides to the H₂S bearing vapor phase greatly enhances the solubility of Au. A vapor phase with 2.65 wt% NaCl and 10.1 wt% S dissolves 1080±120 µg/g Au. Replacement of Na by K maintaining the same Cl and S concentration results in a further 3 fold increase in gold solubility to 2880±430 µg/g. The above observations can be explained by the following type of dissolution reaction:

Au + KCl + 2H₂S = KAu(HS)₂ + HCl + ½ H₂

where K can be substituted by Na. Our observation that the presence of alkali metals greatly increases the affinity of Cl and S to form complexes with Au in high-temperature magmatic volatiles is consistent with the fact the many giant Au-rich porphyry and epithermal Au deposits occur in the vicinity of alkaline mafic intrusions, particularly of those with potassic composition (Müller and Groves, 1993).

Müller and Groves (1993) Ore Geol Rev 8, 383-406.