Phase relations and the electrochemical determination of sulfur fugacity for selected reactions in the Cu–Fe–S and Fe–S systems at 1 bar and temperatures between 185 and 460 °C

Almost 30 years ago, Schneeberg [Econ. Geol. 68 (1973) 507] used the Ag/AgI/Ag2+xS, fS2(vapor) electrochemical cell for directly measuring the fugacity of sulfur vapor in equilibrium sulfide reaction mixtures in the Cu–Fe–S, Ni–S and Fe–S systems at temperatures between 210 and 445 °C. The present study has re-examined three of the reactions investigated by Schneeberg (1973), and expanded this work to include four others in the low-sulfur portion of the Cu–Fe–S system. fide ore assemblages the abundant iron sulfides pyrite (py) and/or pyrrhotite (po) are commonly associated with chalcopyrite (cp) and sphalerite. Other Cu–Fe–S sulfides, including covellite (cv), idaite (id), bornite (bn) and cubanite (cb), may also occur, but are less common because they require relatively high-sulfur or low-sulfur conditions in order to form. The range of logfS2–temperature conditions that apply to most ore assemblages is therefore contained by reactions that involve these minerals. Accordingly, sulfur fugacities were determined for the following reactions: (1) py+Id=bn+S (sulfur vapor), (2) py+bn=cp+S, (3) py+cp=cb+S, (4) po+cp=cb+S, (5) py+cb=po+cb+S, (6) py+cp=po+cp+S and (7) py=po+S. sults for reaction (1) indicate lower sulfur fugacities than were obtained by Schneeberg (1973). For reactions (2) and (7), the present results show excellent agreement with those of Schneeberg (1973) at higher temperatures. However, at lower temperatures, previously undetected inflections in logfS2 versus 1000/T plots occur at ∼221 and ∼237 °C for reactions (1) and (2), respectively. An invariant point occurs at ~335 °C and a sulfur fugacity of ∼10−9 where the phases py, cp, cb and po coexist with sulfur vapor. These results confirm earlier findings that reactions (3) and (5) occur above the invariant temperature and are replaced by reactions (4) and (6) below it. For the pyrite=pyrrhotite+S reaction (7), a previously unidentified inflection occurs at ∼291 °C. The latter implies a phase transition in hexagonal pyrrhotite that appears to coincide with the lower temperature boundary of the sphalerite geobarometer. The present data provide a logfS2–T framework that will facilitate future calibration of the FeS content in sphalerite.