Most of the world's rhenium (~90%) is obtained from molybdenite (MoS2) of porphyry deposits. Paradoxically, the highest Re contents in MoS2 are observed in Cu and Cu-Au porphyry systems with smaller concentrations in Cu-Mo, Mo and W-Mo deposits (Barton et al., 2020). The goal of this study is to determine the geochemical mechanisms that lead to specific Re enrichments in different types of porphyry deposits.

We combined thermodynamic modeling with solubility experiments in H2O-salt-sulfur systems to assess the role of salt and sulfur ligands in the behavior of Re and other metals in supercritical fluids. Our analysis of available data shows that the presence of salt and sulfur may have contrasting impacts on metals transport: a) soft metals (Au) form stable complexes with Cl- and S-bearing ligands; b) hard metals (W, Mo) form oxyanions and their ion pairs with Na and K; however, in sulfur-rich fluids Mo and W also form thiomolybdates and thiotungstates; c) Re is a borderline metal similar to Cu that exhibits characteristics of both hard and soft cations depending on its oxidation state and coordination environment. Taking into account the high affinity of Re to Au-Cu porphyry ores, rhenium should be best transported by complexes with chloride and sulfide anions in its oxidation state of IV. 

We are currently investigating the solubility of major ore minerals in supercritical fluids in the presence of mineral buffers of pH, fS2 and fO2. These data will allow a revision of the existing speciation models for critical metals in hydrothermal fluids. In addition to the traditional ligands, the trisulfur radical ion may also be an important carrier of Mo and Re in hydrothermal fluids that is a subject of our on-going work. 

Barton, I. F., Rathkopf, C. A., and Barton, M. D. (2020) Min. Metall. Explor., 37, 21-37.