MULTIPLE SULFUR ISOTOPES OF PALEOPROTEROZOIC C- AND S-RICH METASEDIMETARY ROCKS ASSOCIATED WITH SULFIDE ORE DEPOSITS IN EASTERN FINLAND AND NORTHWESTERN RUSSIA

Previous geochemical studies of Archean-Paleoproterozoic sedimentary rocks older than 2.4 Ga have documented large anomalous fractionations of sulfur isotopes (Δ³³S= >0±0.2‰), while younger rocks record little to no anomalous fractionation of sulfur isotopes. This change from large anomalous fractionations to sulfur isotope values that fall on the ”terrestrial fractionation line” (TFL) have been proposed to represent the transition from the anoxic Archean atmosphere to an oxygenated atmosphere. Here we present δ³⁴S and δ³³S data for eleven core samples from Paleoproterozoic (1.9 Ga) black shales and associated sulfide ores in eastern Finland and northwestern Russia. Previous δ³⁴S of studies of the Talvivaara and Outokumpu deposits of eastern Finland, have focused on single grain analyses of sulfides, while this study presents data from sequentially extracted sulfur fractions (i.e., acid soluble sulfides, chrome reducible sulfides, elemental sulfur, acid soluble-sulfate). Our δ³⁴S, δ³³S, and Δ³³S results range between +16.6‰ to -11.6‰, +8.6‰ to -6.0‰, and +0.32 to -0.36 respectively. We report a wider range of Δ³³S values than previously reported for sulfides from Talvivaara and Outokumpu ore deposits in eastern Finland (Papineau et al., 2005), the δ³³S/ ^δ34S value for our samples, 0.520, is in good agreement with the Papineau et al. (2005) δ³³S/ δ³⁴S value of 0.518. Additionally, our data are consistent with previous findings that large anomalous fractionations of sulfur isotopes are absent in rocks younger than 2.4 Ga.

Although some samples presented here show what may be small anomalous fractionations (i.e., beyond 0±0.2‰) that could indicate either ultraviolet photolysis of volcanic SO₂ in an oxygen-poor atmosphere or thermochemical sulfate reduction in the presence of organic matter and hydrothermal sulfate-rich solutions. However, it is clear that further sulfur isotopic analyses are needed to test the fidelity of these small anomalous fractionations. The black shales and associated ore deposits in eastern Finland have been interpreted as an ophiolite sequence and the sulfur isotope data suggest bacterial reduction of seawater sulfate along with hydrothermal sulfur inputs.