• journal_title：Economic Geology
• Contributor：Michael J. Mengason ; Philip M. Piccoli ; Philip Candela
• Publisher：Society of Economic Geologists
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2113/econgeo.105.6.1163
• journal_abbrev：Economic Geology
• issn：0361-0128
• volume：105
• issue：6
• firstpage：1163
• section：Scientific Communication

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 <span id="xref-ref-22-1" class="xref-bibr">Toulmin and Barton (1964)span>, which is commonly used to determine sulfur fugacity from the composition of binary pyrrhotite (Fe<sub>1−xsub>S) 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 <span id="xref-ref-22-2" class="xref-bibr">Toulmin and Barton (1964)span> 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 N<sub>FeSsub> in the equation of <span id="xref-ref-22-3" class="xref-bibr">Toulmin and Barton (1964)span>, calculated as X<sub>FeSsub><sup>pyrrhotitesup> in the system FeS-S<sub>2sub>, was replaced by terms that incorporated the effect of copper. Method 1: <span class="inline-formula" id="inline-formula-1"> ss="math mml" alt="Formula" src="1163/embed/mml-math-1.gif" />span>—copper was ignored. Method 2: <span class="inline-formula" id="inline-formula-2"> ss="math mml" alt="Formula" src="1163/embed/mml-math-2.gif" />span>—copper was treated as CuS<sub>0.5sub>. Method 3: <span class="inline-formula" id="inline-formula-3"> ss="math mml" alt="Formula" src="1163/embed/mml-math-3.gif" />span>—copper was treated as CuS. Method 1 overestimated log f<sub>S<sub>2sub>sub> (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 f<sub>S<sub>2sub>sub> 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 f<sub>O<sub>2sub>sub> per wt percent copper by the use of either Method 1 or 3 in determining sulfur fugacity.