The Hormuz Island banded iron formations (HIBIFs) are situated at the Zagros Folded Zone in southern Iran. The BIFs occur intercalated with the Neoproterozoic Hormuz series, an evaporite and siliciclastic succession with pyroclastic rocks, and were deposited on the continental rift margin of the Arabian plate. The HIBIFs are banded rocks consisting of the alternation of dark Fe-rich and reddish-brown Si-rich layers, and belong to oxide facies iron formations. The post-Archean Australian shale normalized rare earth elements and yttrium patterns of HIBIFs have the characteristics of both hydrothermal solutions and seawater. The sharp positive Eu anomaly (2.10–6.90) may be inherited from high-temperature hydrothermal solutions, while slightly negative Ce anomalies (0.57–0.82) are inherited from seawater. The homogenization temperature and salinity of primary fluid inclusions range from 180.7 to 474.3°C and from 2.24 to 12.85 wt% NaCl, respectively, which suggests that intermediate- to high-temperature hydrothermal fluids and seawater contributed to the formation of the HIBIFs. The Eu/Sm and Sm/Yb ratios in BIFs are between the values of hydrothermal fluids and seawater. In summary, all geological and geochemical data suggest that the HIBIFs are a post-great oxidation event iron formation pulse at the end of the Ediacaran period. However, this is actually not similar to Neoproterozoic syn-glacial iron formations. Instead, this is a new occurrence of iron formation in an overlooked/understudied time period when either global or local conditions were conducive to iron deposition. The iron deposition on Hormuz Island does not appear to be triggered by fluctuating atmospheric oxygen or sea-level variations, but instead may be related to syn-basinal volcanism in the active continental rift margin environment in anoxic submarine condition.

Thematic collection: This article is part of the Geochemical processes related to mined, milled, or natural metal deposits collection available at: https://www.lyellcollection.org/topic/collections/geochemical-processes-related-to-mined-milled-or-natural-metal-deposits

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