MULTIPLE SULFUR ISOTOPES OF PALEOPROTEROZOIC ROCKS IN EASTERN FINLAND: POTENTIAL EVIDENCE FOR THE THERMOCHEMICAL SULFATE REDUCTION PATHWAY TO MIF-SULFUR

Previous stable isotope studies of Archean-Paleoproterozoic sedimentary rocks older than 2.4 Ga have documented large mass-independent fractionations (MIF) 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 twenty-eight core samples from Paleoproterozoic (1.9 Ga) metasedimentary black shales and associated sulfide ores in eastern Finland. 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 (e.g., acid soluble sulfides, chrome reducible sulfides, elemental sulfur). The sulfur isotope results range between: +16.6‰ to -11.6‰ for δ³⁴S, +8.6‰ to -6.0‰ for δ³³S, and +1.25 to -0.55‰ for Δ³³S. We report a wider range of Δ³³S values than previously reported for sulfides from Talvivaara and Outokumpu ore deposits in eastern Finland, with the δ³³S/ δ³⁴S value for the majority of our samples being near 0.515, although some of our samples show values from 0.393 to 0.644. These anomalously fractionated samples are not consistent with previous findings that large anomalous fractionations of sulfur isotopes are absent in rocks younger than 2.4 Ga. Anomalous fractionations recorded here could be the products of ultraviolet photolysis of volcanic SO₂ in an oxygen-poor atmosphere. Alternatively, these anomalous fractionations in sulfur isotopes recorded here could be the result of thermochemical sulfate reduction in the presence of organic matter and hydrothermal sulfate-rich solutions (Watanabe et al., 2009), which is consistent a sedimentary-volcanic exhalative model proposed for the origin of the S- and C_org-rich metasedimentary rocks associated with the Outokumpu ore deposits. This is the first geologic evidence supporting this alternative pathway for MIF-Sulfur. Although hydrothermal fluid interactions with older (>2.4 Ga) sedimentary crustal rocks could be plausible for these sulfur isotopic signatures.