Pyritic event beds and sulfidized Fe (oxyhydr)oxide aggregates in metalliferous black mudstones of the Paleoproterozoic Talvivaara formation, Finland

Highlights

• Talvivaara formation was deposited in a river and tidally influenced coastal sea area.

• The coastal waters had a shallow oxic surface layer underlain by euxinic water.

• Excursions of the acidic Fe-rich river plume deposited pyrite beds on distal muds.

• Fe (oxyhydr)oxide aggregates were transported offshore by muddy flows and pyritized.

• $\delta {}^{34}\mathrm{S}$ and $\delta {}^{56}\mathrm{Fe}$ indicate syndepositional to shallow-burial origin of Fe-sulfides.

Abstract

The Paleoproterozoic, 2.0–1.9 Ga Talvivaara formation of Finland was deposited during the Shunga Event, a worldwide episode of enhanced accumulation of organic-rich sediments in the aftermath of the Lomagundi–Jatuli carbon isotope excursion. Sulfidic carbonaceous mudstones in the Talvivaara formation contain one of the largest known shale-hosted nickel deposits. In order to gain new insight into this Shungian sedimentary environment, sedimentological, petrographical and in situ S and Fe isotopic microanalyses were carried out on samples representing depositional and early-diagenetic conditions. The event-bedded lithology with tidal signatures in the organic-rich mudstones strongly indicates deposition from predominantly river-delivered mud on a highly-productive coastal area, below storm-wave base. The riverine supply of phosphorus, sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water. The stratigraphic upper part of the Talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds. The pyrite beds were deposited by seaward excursions of the concentrated, acidic Fe-rich river plume subsequent to droughts or dry seasons, which led to intense pyrite precipitation upon mixing with euxinic waters. $\delta {}^{34}\mathrm{S}$ and $\delta {}^{56}\mathrm{Fe}$ values of the bedded pyrite (median $\delta {}^{34}\mathrm{S}=-10.3\mathrm{‰}$ and $\delta {}^{56}\mathrm{Fe}=-0.79\mathrm{‰}$) are consistent with the reaction of dissolved Fe(II) with H₂S from bacterial sulfate reduction. Organic-rich clayey Fe-monosulfide-bearing granules were transported from the muddy estuary, and enclosed in Fe (oxyhydr)oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river-delivered iron. The isotopic composition of these presently pyrrhotitic inclusions (median $\delta {}^{34}\mathrm{S}=-3.3\mathrm{‰}$ and $\delta {}^{56}\mathrm{Fe}=-1.6\mathrm{‰}$) indicates microbial iron reduction. The Fe (oxyhydr)oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor. Their Fe (oxyhydr)oxide matrix was replaced by pyrite (median $\delta {}^{34}\mathrm{S}=+5.8\mathrm{‰}$ and $\delta {}^{56}\mathrm{Fe}=+0.81\mathrm{‰}$) at shallow sediment depths with ³⁴S and ⁵⁶Fe-enriched porewater. Wavy-crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation. These results indicate episodic deposition at seasonal to multiannual time scales. $\delta {}^{34}\mathrm{S}$ and $\delta {}^{56}\mathrm{Fe}$ values in the studied Fe-sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early-diagenetic origin, finding no support for the previously proposed local hydrothermal activity in the Talvivaara mudstones.

Introduction

Earth's ocean–atmosphere system saw marked changes in the early Paleoproterozoic between 2.5 and 2.0 Ga ago. A significant rise in the atmospheric oxygen concentration at $\sim 2.3\text{ Ga}$ accelerated the oxidative weathering of subaerial lithologies and sulfide minerals (Great Oxidation Event; Holland, 2002, Konhauser et al., 2011). This liberated vast quantities of phosphorus and nutrients to the oceans, stimulating primary productivity and resulting in high rates of organic carbon burial as recorded by the Lomagundi–Jatuli positive carbon-isotope excursion 2.2 to 2.06 Ga ago (Karhu and Holland, 1996, Papineau, 2010, Melezhik et al., 2013). The Lomagundi–Jatuli was followed by an unprecedented accumulation of organic-rich sediments worldwide during the Shunga Event (Strauss et al., 2013). The Shunga Event likely resulted from excessive primary productivity and/or favorable conditions for the burial of an exceptionally large share of the produced organic matter. A peak in continental rifting at $\sim 2.0\text{ Ga}$ favored the accumulation and preservation of carbonaceous strata at the time (Lahtinen et al., 2010, Melezhik and Hanski, 2013). Weathering and oxidation of the deposited organic carbon may have led to a relapse in atmospheric oxygen levels for some 200 Ma after the Lomagundi–Jatuli (Canfield et al., 2013).

The global ocean chemistry during the early Paleoproterozoic is still debated, but oxygenic photosynthesis by cyanobacteria in nearshore areas likely resulted in a shallow oxic surface layer, underlain by either ferruginous (anoxic with dissolved iron, Fe(II)_aq) or euxinic (anoxic with dissolved H₂S) water (Fralick and Pufahl, 2006, Poulton and Canfield, 2011, Reuschel et al., 2012, Planavsky et al., 2012, Canfield et al., 2013, Pufahl et al., 2014, Kontinen and Hanski, 2015). Riverine delivery of sulfate and the resulting sulfate reduction led to the precipitation of Fe(II)_aq by H₂S as sulfide minerals, and to at least locally euxinic coastal waters (Poulton and Canfield, 2011, Planavsky et al., 2012).

The iron sulfide-rich, carbonaceous mudstone-dominated Talvivaara formation in eastern Finland (Fig. 1) was deposited in a narrow intracratonic marginal basin after the breakup of the late Archean supercontinent Kenorland, between 2.1 and 1.95 Ga (Lahtinen et al., 2010, Melezhik and Hanski, 2013, Kontinen and Hanski, 2015). The post-breakup strata are characterized by turbidites, and lack volcanic intercalations (Kontinen and Hanski, 2015). The Talvivaara formation is broadly contemporaneous with the highly organic-rich sedimentary rocks of the 2.09–1.98 Ga Zaonega Formation in Russian Karelia that include the Shunga Event type locality (Strauss et al., 2013), and with the $\sim 2.08\text{ Ga}$ euxinic black shales of the Francevillian basin, Gabon (Canfield et al., 2013).

Previous studies on the origin of the Talvivaara formation are mainly based on bulk geochemistry and mineralogy. High contents of sulfur (2.4–25.3 wt.%) and organic carbon (2.2–13.2 wt.%) and $\delta {}^{13}\mathrm{C}_{\text{org}}$ values of $-24\text{ to }-27\mathrm{‰}$ PDB were interpreted to reflect deposition on an anoxic/euxinic seafloor of a highly productive, restricted stratified deep-water basin, with iron sulfide $\delta {}^{34}\mathrm{S}$ data indicating both bacterial and thermochemical sulfate reduction (Loukola-Ruskeeniemi and Heino, 1996, Loukola-Ruskeeniemi and Lahtinen, 2013, Young et al., 2013). Young et al. (2013) argued, based on anomalously fractionated $\mathrm{\Delta }{}^{33}\mathrm{S}$ values, for thermochemical reactions during interaction of sulfate-rich hydrothermal fluids with the Talvivaara muds. Strong enrichment in redox-sensitive metals such as Fe, Mo, Se, V, Cd and U were attributed to a local hydrothermal source, whereas the source for base metals Ni and Cu, and potentially Zn, was thought to be ultramafic igneous rocks (serpentinites) in the area (Loukola-Ruskeeniemi and Heino, 1996, Loukola-Ruskeeniemi and Lahtinen, 2013). The hydrothermal input in metal-enrichment has been questioned, however, because the base metal ratios (e.g. Zn/Ni and Cu/Ni) support hydrogenous metal scavenging processes and unrestricted connection to the global ocean (Laitala, 2014, Kontinen and Hanski, 2015).

Recent studies have paid more attention to sedimentological and petrographical properties of the Talvivaara formation. The formation is mainly composed of poorly-graded mudstone beds with local semidiurnal tidal signatures, and minor quartz-sand interbeds with erosional bases and normal grading that attest to episodic mass flow-deposition on a basin margin (Laitala, 2014, Kontinen and Hanski, 2015). The stratigraphic upper part of the Talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds that reflect reduced detrital supply and increased chemical deposition (Laitala, 2014, Kontinen and Hanski, 2015). The pyritic bedding is very similar to the 1.88 Ga Wauseca Pyritic Member of Dunn Creek Slate, Animikie Basin, Lake Superior region (James et al., 1968, Rouxel et al., 2005).

This paper examines the microscale S and Fe isotope composition of in-situ iron sulfide minerals in sedimentologically and petrographically well-characterized samples from sulfide-rich carbonaceous mudstones in the upper part of the Talvivaara formation. The sample materials are from low-strain sections, where the primary sedimentary features are well preserved. The purpose is to gain new high-resolution insight into the physical, chemical and microbiological processes that were involved in the deposition and early diagenesis of the Talvivaara formation. The results place important constraints on the origin of primary metal enrichment in the Talvivaara formation, and significantly add to the understanding of surficial drainage processes and river-influenced coastal sedimentary environments during the global Shunga Event.