EXPERIMENTAL STUDY OF TELLURIUM IN CHLORIDE BRINES

The chemistry of tellurium is complex due to the numerous oxidation states available for this element. Thus an extensive knowledge of the thermodynamic properties of the different Te-species is required to be able to build accurate models of the chemistry happening in Te-bearing hydrothermal systems, and eventually to solve important questions such as the link between gold and tellurium in many gold deposits.

Previous studies have mainly focused on the lower oxidation states of Te as they are those found in most hypogene Te-bearing minerals. Despite these efforts the amount of data available is still limited and fails to predict solubilities of Te which are in agreement with actual observations (McPhail, Geochim. Cosmochim. Acta 1995, 59, 851-866; Wallier et al., Economic Geology 2006, 101, 923-954).

Most probably an important feature escaped the attention of previous investigators. By reassessing the stability fields of the different Te species in the pH-f(O₂) space we noted that the stability of aqueous Te(IV) species vs Te(-II) species increases with temperature. According to this analysis the stability domain of Te(IV) at 300°C may be much closer to the hematite/magnetite buffer than believed previously. However, these predictions suffer from very high uncertainty because they are based on extrapolations of thermodynamic properties available at 25°C (and often with large errors even at 25°C !).

We therefore decided to investigate the chemistry of the Te(IV)-Cl-H₂O system experimentally over a wide concentration range of chloride and acid, and up to 95°C, using a combination of UV-Vis spectrophometry, ¹²⁵Te NMR spectroscopy and XAS. This combined information provides the first insight into the complexity of hydrothermal geochemistry of tellurium under mild hydrothermal conditions.