Reactions of hydrothermal solutions with organic matter in Paleoproterozoic black shales at Talvivaara, Finland: Evidence from multiple sulfur isotopes

Abstract

Stable isotopic studies of Archean–Paleoproterozoic sedimentary rocks older than 2.4Ga have documented large mass-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 has been proposed to represent the transition from an essentially anoxic Archean atmosphere to an oxygenated atmosphere. Here we present δ34S and δ33S data for 28 core samples from Paleoproterozoic (2.1–1.9Ga) metamorphosed black shales and associated sulfide ores in eastern Finland. Previous δ34S of studies of the Talvivaara and Outokumpu deposits of eastern Finland 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 and −11.6‰ for δ34S, +8.6 and −6.0‰ for δ33S, and +1.25 and −0.55‰ for Δ33S. The anomalously fractionated samples are not consistent with previous findings that large anomalous fractionations of sulfur isotopes are absent in sedimentary/metasedimentary rocks younger than 2.4Ga. It is unlikely that Talvivaara sulfides are the products of ultraviolet photolysis of volcanic SO2 in an oxygen-poor atmosphere. Alternatively, these sulfides could in part be the products of diagenetic reactions between sediments enriched in organic matter (Corg) and hydrothermal solutions rich in sulfate (i.e., thermochemical sulfate reduction). Laboratory experiments on thermochemical sulfate-reduction have shown that mass-anomalous fractionations of sulfur isotopes can be recorded in reduced-sulfur products from reactions between simple amino acids and sulfate at 150–300°C. A thermochemical sulfate-reduction pathway for the mass-anomalous signatures in Talvivaara samples is consistent with previous genetic models proposed for the origin of the sulfur- and organic carbon-rich black shales (now schists) associated with the Talvivaara and Outokumpu ore deposits. At Talvivaara metalliferous black shales apparently were deposited under an anoxic to sulfidic marine water column. The reported multiple sulfur-isotopic data are key geological evidence for mass-anomalous fractionations of sulfur isotopes during hydrothermal alteration of fine-grained organic-rich sediments. Approximately 20% of samples in this study show large offsets in Δ33S values between different extracted sulfur fractions from the same sample, which can be explained by classical isotope effects associated with the formation of hydrothermal alteration products that predominantly are retained within the precursor fine-grained organic-rich units.