Une étude expérimentale du transport de Cu, Ag et Au par vapeur

Projet DIVEX SC18

Responsable du projet: A.E. William-Jones, McGill University, courriel
Collaborateur: Zezin D. Yu, McGill University; Migdisov Art. A., McGill University

Introduction

Existing models for the formation of porphyry Cu and epithermal Au-Ag deposits have been built on the assumption that the metals are transported to the site of deposition by an aqueous liquid. However, there is strong evidence that magmatic vapors may contain significant concentrations of Cu, Au, and Ag, and it is therefore possible that the vapor phase may play an important and even dominant role as an agent of metal transport in porphyry/epithermal ore-forming systems.

Objectives

The long term objective of the research described in this report is to contribute some of the key thermodynamic data needed to quantitatively test the vapor transport model of porphyry/epithermal ore genesis and to build on previous work in the system H₂O-HCl. Noting that H₂S partitions strongly into the vapor phase and that H₂S has been shown to form strong complexes with Cu, Au and Ag, the current research is investigating the solubility of Cu, Au, and Ag in H₂S vapor.

Method

The method used in this study was similar to that described in our previous studies devoted to the vapor transport of metals (e.g., Migdisov et al, 1999, Archibald et al., 2001 and 2002). The experiments were carried out in titanium-alloy autoclaves (with volume of ~40-50 cm³), which were preconditioned with nitric acid to produce a protective layer of TiO₂ on the internal surfaces.

Schematic sketch of the experimental set-ip

Results and conclusions

The behavior of Cu, Au, and Ag was studied experimentally at temperatures between 200 and 400°C and established that metal solubility increases with increasing temperature and that H₂S gas can dissolve up to 7.5 ppm Cu, up to 150 ppb Ag, and up to 37 ppb of Au at temperatures of 300, 350 and 400°C, respectively. The species responsible for the transport of Ag and Au were determined to be solvation complexes of the type Ag₂S·(H₂S)^gas (T~350°C) and Au·(H₂S)^gas (T~400°C) having statistical solvation numbers of approximately 1 at 350°C and 1.6 at 400°C respectively. The relatively high solubility of Cu and its apparent independence of ƒH₂S are tentatively attributed to an unusually high volatility of covellite, the copper reagent employed in the experiments. The results of this experimental study suggest that H₂S may be very important in enhancing the solubility of Ag and Au metals in the vapor phase, and will add important data to the framework that will be needed to permit construction of quantitative models of the vapor transport of metals in natural systems.

Références

ARCHIBALD, S.M., MIGDISOV, A.A., WILLIAMS-JONES, A.E. (2001) The stability of Au-chloride complexes in water vapor at elevated temperatures and pressures. Geochim. Cosmochim. Acta 65, 4413–4423.

ARCHIBALD, S.M., MIGDISOV, A.A., WILLIAMS-JONES, A.E. (2002) An experimental study of the stability of copper chloride complexes in water vapor at elevated temperatures and pressures. Geochim. Cosmochim. Acta 66, 1611–1619.

MIGDISOV, A.A., WILLIAMS-JONES, A.E., SULEIMENOV, O.M. (1999) The solubility of chlorargyrite (AgCl) in water vapor at elevated temperatures and pressures. Geochim. Cosmochim. Acta 63, 3817-3827.