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Abstract

The SIMS δ 34 S study of pyrite from the late Devonian shale - hosted volcanogenic massive sulfide deposit of Sotiel - Migollas (Iberian Pyrite Belt) combined with the 87Sr / 86Sr ratio of the intergrown carbonates [1] track the complex evolution of the seafloor hydrothermal system. SIMS δ 34 S of pyrite showing different textures exhibit a wide range in values from - 35.8±4.6‰ in the primary framboidal morphologies to - 0.9±1.3‰ in the late recrystallized euhedral crystals. The 87 Sr/ 86 Sr ratios vary between 0.70846 and 0.70983, being the less radiogenic values close to those of the coeval seawater, estimated by [2] in the 0.70750 - 0.70850 range. There is a strong linear correlation between these two isotope systems, with the most negative δ 34 S values of the pyrite being intergrown with carbonates with the lowest 87 Sr/ 86 Sr ratios. Extremely 34 S - depleted values can only be interpreted as reflecting the biogenic reduction of the seawater sulfate with a large isotopic fractionation in an open system [3]. This is consistent with a strontium seawater signature, suggesting that these samples correspond to the external parts of the exhalative system in contact with seawater and with bacterial sulfate reduction taking place on the seawater - massive sulfides interface. In contrast, the other endmember is interpreted as reflecting the late maturation of the system inside the ore lens with input of deep sulfur - likely derived from TSR processes or leached from the basement - accompanied by 87 Sr - enriched fluids. We propose a model for the growth of biogenic mounds on the seafloor with alternating carbonate - sulfide bands that were gradually buried by subsequent growth. The isotope signatures changed due to the later maturation of mound and the input of deep hydrothermal fluids.