REACTION PATH MODELING OF SULFUR ISOTOPE FRACTIONATION DURING PROGRESSIVE PYRITE REPLACEMENT OF HEMATITE, SUPERIOR C-BED CU-AU ORE BODY, ARIZONA

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 δ³⁴S_pyrite 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 Fe₂O_3(hematite) + 2 H^+} + 15 H₂S + SO₄^-2} = 8 FeS_2(pyrite) + 16 H₂O

resulted in a sharper decrease in fluid H₂S:SO₄ ratios, but nearly constant calculated δ³⁴S_pyrite 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 δ³⁴S_pyrite 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 δ³⁴S_mixture values of pyrite with anhydrite contamination. No mixture of anhydrite and pyrite resulted in calculated δ³⁴S_mixture 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.