Lithological and geochemical evidence of Fe and Mn pathways during deposition of Lower Proterozoic banded iron formation in the Krivoy Rog Basin (Ukraine)

Abstract

Data on stratigraphy, chemical and mineral composition of siliciclastic and ferrugionus rocks of greenschist to epidote-amphibolite metamorphic grade, multi-order cyclicity and banding, isotopic composition of sulphur and carbon and other features of the Krivoy Rog Supergroup metasedimentary rocks are summarized. They provide an insight into the primary sources and the nature of siliciclastic and chemogenic components of banded iron formation (BIF), as well as on pathways of Fe, Mn and Si in the Krivoy Rog palaeobasin at the time of BIF accumulation. Skelevat and Saxagan iron-ore Groups were deposited in a single cyclic transgressive-regressive sequence, probably in a trough-like landlocked basin with complex bottom topography defined by syn-sedimentary faults inherited from the earlier rift structures. Variations in sea level, humidity of climate, continental runoff, oceanic water inflows etc. many times provoked the rapid facies changes ranging from sedimentation of land-derived siliciclastics to deposition of nearly pure chemogenic sediments of the oxide facies BIF in the deeper parts of the basin. Up to seven pairs of schist (s-) and BIF (f-) horizons were deposited in the Saxagan Group. Lateral variations of their thicknesses, distribution of free carbon in the rocks and other data indicate that only parts of the basin at a given time were optimal for chemical sedimentation; major part of dissolved Fe, Si and Mn was supplied by constant or event inflow of suboxic ocean water.The Mn content in the Krivoy Rog Supergroup BIFs could serve as an indicator of the fraction of total Fe that precipitated chemically as carbonate. The main part of Fe precipitation in form of Fe(III) hydroxide particles, possibly coated with amorphous silica, is due to a number of oxidation mechanisms operating in the photic surface water layer. On the basis of physicochemical considerations, it is concluded that the Mn CO3 content in siderite (c. 2mol %) contained in the BIF rocks reflects a uniform dissolved Mn concentration (c. 1 ppm) in the ocean and basin waters. Sedimentation of the typical association of Mg-Fe carbonates and silicates of the carbonate-silicate facies BIF could occur in the near-bottom water only in the case of strong enrichment in dissolved Fe(II) (up to 100-200ppm) relative to the ocean water (1-10ppm), which required a stratified brackish-water basin with an estuarine-type circulation. No enrichment in dissolved Mn(II) was possible until Po2 in the atmosphere raised high enough for precipitation of particulate Mn oxide in the photic zone and its dissolution in the suboxic near-bottom waters.

Geochemistry of the Krivoy Rog Banded Iron Formation, Ukraine, and the impact of peak episodes of increased global magmatic activity on the trace element composition of Precambrian seawater

“…The Fe content of the lower sequences is less than 40%, while upper Saxagan Group BIF horizons may contain up to 65% Fe (Mboudou et al, 2012), probably triggered by post-depositional supergene Fe-ore upgrade. Mboudou et al (2012) and Belevtsev and Belyaev (1989) report greenschist facies metamorphic conditions in the lower part of the Saxagan BIF stratigraphy which increase to amphibolite facies in the upper part of the sequence and are probably related to a Kulik and Korzhnev, 1997). regional granitoid emplacement at 2000 ± 100 Ma (Shcherbak et al, 1984). However, all BIF samples we studied are mainly composed of magnetite and quartz with minor siderite, hematite and low-grade metamorphic trace minerals such as riebeckite, cummingtonite, stilpnomelane and chlorite minerals.…”

“…The Konka and the Belozerka Supergroup are comprised of metavolcanics, rare metasediments and various intruded TTGs which yield U-Pb zircon ages between 3120 ± 10 Ma and 2815 ± 15 Ma (Shcherbak et al, 1989;Samsonov and Chernyshev, 1996;Bibikova et al, 2010, and references therein). The stratigraphically younger Late Archean to Lower Proterozoic greenschist-to amphibolite-facies Krivoy Rog Supergroup is divided into five subgroups (after Belevtsev and Belevtsev, 1981;Belevtsev et al, 1983 andsummarized in Kulik andKorzhnev, 1997;Fig. 1).…”

“…The Krivoy Rog Supergroup comprises seven cycles of clastic and chemical sediments with a total thickness of up to 1400 m (Belevtsev et al, 1983) and a lateral extent of 120 km × 10 km. Drastic sea level changes of the "Krivoy Rog sea" in the tectonically unstable, restricted Krivoy Rog trough characterized the depositional environment (Belevtsev et al, 1983;Kulik, 1991;Kulik and Korzhnev, 1997). Post-depositional lower greenschist-to amphibolite-facies tectono-metamorphic events and supergene weathering eventually produced the economically important iron ores of the Krivoy Rog area (Kulik and Korzhnev, 1997) from a BIF precursor.…”

Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

“…Deposition of these iron formations occurred on a reactivated continental margin (Krapež et al, 2003), during a rise in sea level. In addition to these time-equivalent iron formations, those in the Quadrilátero Ferrífero region in Brazil, Krivoy Rog area in Ukraine, and Kursk Magnetic Anomaly region in Russia are broadly similar in age based on available geochronologic and chemostratigraphic constraints, as well as on similar patterns of megabanding (Prilutzky et al, 1992;Kulik and Korzhnev, 1997;Bekker et al, 2003;Spier et al, 2007). These iron formations were also deposited on reactivated continental margins and are separated by a prominent unconformity from overlying Paleoproterozoic sequences.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…Iron formations in the Quadrilátero Ferrífero region in Brazil, Middleback Ridge (Gawler Craton) in South Australia, Nimba and Simandou Ranges in Liberia and Guinea, Krivoy Rog area in Ukraine, and Kursk Magnetic Anomaly (KMA) region in Russia are broadly similar in age based on available geochronological and chemostratigraphical constraints, and also display similar patterns of macrobanding (Prilutzky et al, 1992;Kulik and Korzhnev, 1997;Egal et al, 2002;Bekker et al, 2003;Phillips et al, 2006;Spier et al, 2007;Szpunar et al, 2011). These IFs were also deposited on reactivated continental margins, and are separated by prominent unconformities from overlying Palaeoproterozoic sequences, which correspond to a long gap in sedimentation following the supercontinent assembly at ca.…”