GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 65-14
Presentation Time: 9:00 AM-5:30 PM


MAGUIRE, Sara E. and FRIEHAUF, Kurt, Dept. Physical Sciences, Kutztown University, Kutztown, PA 19530,

Measured sulfur isotope compositions of pyrite in the carbonate-hosted massive replacement Cu-Au ores in Superior, Arizona have a spatial pattern of systematically heavier δ34S values increasing from -4.8‰ in the feeder vein, to 0.0‰ at the interface between massive pyrite and massive specular hematite, to +7.8 to +10.1‰ in hematite-dominant ore 75 meters away. We incrementally calculated the δ34S values of H2S, SO4-2, and pyrite formed by progressive reactions between early-stage hematite and an initial 300⁰C H2S-dominant fluid in equilibrium with feeder vein pyrite of δ34S = -4.8‰.

Simple isothermal sulfidation of hematite by H2S at constant SO4-2 concentration gradually decreased the H2S:SO4 ratio of the fluid as pyrite deposited H2S, resulting in progressively lighter pyrite further from the vein, with calculated δ34Spyrite decreasing to -7.7‰ at H2S exhaustion, which is opposite the observed, measured pattern in the ore body.

Isothermal sulfidation of hematite by H2S and SO4-2 according to the reaction:

4 Fe2O3(hematite) + 2 H+} + 15 H2S + SO4-2} = 8 FeS2(pyrite) + 16 H2O

resulted in a sharper decrease in fluid H2S:SO4 ratios, but nearly constant calculated δ34Spyrite throughout the reaction progress because heavy 34S from SO4 depositing in pyrite balanced light 32S from H2S.

Polythermal cooling from 300⁰ to 205⁰C during sulfidation of hematite by H2S and SO4-2 predicted calculated δ34Spyrite values decrease progressively to -5.6‰ at H2S exhaustion. Partitioning was augmented relative to the isothermal model due to greater fractionation factors at lower temperature.

Because microscopic inclusions of anhydrite have been observed in some pyrite, we calculated the expected δ34Smixture values of pyrite with anhydrite contamination. No mixture of anhydrite and pyrite resulted in calculated δ34Smixture values or trends similar to the observed, measured data.

Simple models of equilibrium fractionation during progressive sulfidation of hematite do not explain the observed isotopic patterns at Superior. Re-equilibration between infiltrating ore fluid and early-formed anhydrite may have contributed 34S to the pyrite-forming reactions in the hematite zone, but not in the inner pyrite zone where porosity was plugged by sulfides and anhydrite was scarce – an untested hypothesis on which we are currently working.