Pyrrhotite is commonly used to estimate the fugacity of sulfur in natural and experimental systems; however, in some instances, high-temperature pyrrhotites can incorporate copper to such an extent as to raise questions concerning the accuracy of the sulfur fugacities calculated on the basis of their composition. The equation of Toulmin and Barton (1964), which is commonly used to determine sulfur fugacity from the composition of binary pyrrhotite (Fe1−xS) solid solutions, can be modified to account for the presence of other phase components. Three methods of incorporating the concentration of copper in the equation of Toulmin and Barton (1964) were evaluated in light of data from experiments performed at 1,000°C, which yielded run-product pyrrhotites with 0.028 (±0.005, 1σ) to 5.75 (±0.06) wt percent copper. Mixtures of synthetic pyrrhotite and bornite were heated in sealed, evacuated silica tubes, which were internally divided into separate chambers by silica rods. As a result, pyrrhotite grains with a range of copper concentrations, as well as copper-free “reference” pyrrhotites, were equilibrated at a common sulfur fugacity. The discrepancy between sulfur fugacity calculated from “reference” pyrrhotite and copper-rich pyrrhotite, based on EPMA analyses, indicated a source of error and potential for disagreement between published accounts when different methods are used to address the presence of copper.

The term NFeS in the equation of Toulmin and Barton (1964), calculated as XFeSpyrrhotite in the system FeS-S2, was replaced by terms that incorporated the effect of copper. Method 1: N=2(nFenFe+nS)—copper was ignored. Method 2: N=2(nFe+nCunFe+1.5nCu+nS)—copper was treated as CuS0.5. Method 3: N=2(nFe+nCunFe+nCu+nS)—copper was treated as CuS. Method 1 overestimated log fS2 (bars) by 0.25 (±0.08, 1σ) per wt percent copper. Method 2 resulted in consistent fugacity estimates regardless of the concentration of copper in pyrrhotite, and is the recommended method. Method 3 underestimated log fS2 by 0.3 (±0.2) per wt percent copper. These systematic errors are propagated into the calculation of oxygen fugacity based on magnetite-pyrrhotite coexistence, resulting in a correction factor of ~¼ log fO2 per wt percent copper by the use of either Method 1 or 3 in determining sulfur fugacity.

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