Articles | Volume 9, issue 3
Research article
04 May 2018
Research article |  | 04 May 2018

Sedimentary mechanisms of a modern banded iron formation on Milos Island, Greece

Ernest Chi Fru, Stephanos Kilias, Magnus Ivarsson, Jayne E. Rattray, Katerina Gkika, Iain McDonald, Qian He, and Curt Broman

Abstract. An early Quaternary shallow submarine hydrothermal iron formation (IF) in the Cape Vani sedimentary basin (CVSB) on Milos Island, Greece, displays banded rhythmicity similar to Precambrian banded iron formation (BIF). Field-wide stratigraphic and biogeochemical reconstructions show two temporal and spatially isolated iron deposits in the CVSB with distinct sedimentological character. Petrographic screening suggests the presence of a photoferrotrophic-like microfossil-rich IF (MFIF), accumulated on a basement consisting of andesites in a ∼ 150 m wide basin in the SW margin of the basin. A banded nonfossiliferous IF (NFIF) sits on top of the Mn-rich sandstones at the transition to the renowned Mn-rich formation, capping the NFIF unit. Geochemical data relate the origin of the NFIF to periodic submarine volcanism and water column oxidation of released Fe(II) in conditions predominated by anoxia, similar to the MFIF. Raman spectroscopy pairs hematite-rich grains in the NFIF with relics of a carbonaceous material carrying an average δ13Corg signature of ∼ −25‰. A similar δ13Corg signature in the MFIF could not be directly coupled to hematite by mineralogy. The NFIF, which postdates large-scale Mn deposition in the CVSB, is composed primarily of amorphous Si (opal-SiO2 ⋅ nH2O) while crystalline quartz (SiO2) predominates the MFIF. An intricate interaction between tectonic processes, changing redox, biological activity, and abiotic Si precipitation are proposed to have collectively formed the unmetamorphosed BIF-type deposits in a shallow submarine volcanic center. Despite the differences in Precambrian ocean–atmosphere chemistry and the present geologic time, these formation mechanisms coincide with those believed to have formed Algoma-type BIFs proximal to active seafloor volcanic centers.

Short summary
Banded iron formations (BIFs) are chemical sediments last seen in the marine sedimentary record ca. 600 million years ago. Here, we report on the formation mechanisms of a modern BIF analog in the Cape Vani sedimentary basin (CVSB) on Milos Island, Greece, demonstrating that rare environmental redox conditions, coupled to submarine hydrothermal activity and microbial processes, are required for these types of rocks to form in the modern marine biosphere.