Banded Iron Travertines at the Ilia Hot Spring (Greece): An interplay of biotic and abiotic factors leading to a modern Banded Iron Formation analogue?

Abstract

A hot spring at Ilia in the Greek Island of Euboea precipitates iron‐rich travertine at an ore‐grade concentration (up to 35.3 wt% Fe). This hydrothermal chemical sediment system deposits bands of iron oxyhydroxides (ferrihydrite), millimetres to centimetres thick, alternating with calcium carbonate‐dominated layers, creating “Banded Iron Travertine” (BIT). The ferrihydrite laminae display a dendritic texture formed of spherical nodules often covering filaments identified as bacterial stalks of Zetaproteobacteria. These microaerophilic iron‐oxidizing bacteria were identified by their 16S rRNA gene sequences in ferrihydrite‐enriched samples from areas under high water flow. They were missing in the aragonite/calcite‐dominated samples exhibiting features of aerial exposure and cyanobacteria instead. These characteristics, and the relative depletion in Fe‐rich layers of redox‐sensitive elements like Mn and Ce, as well as the presence of halite in Ca‐rich layers, suggest that the bands form by successive changes in hydrothermal flow. This allowed microaerophilic iron oxidation to form Fe‐rich layers, while Ca‐rich bands precipitated when the hydrothermal water had time to equilibrate with the atmosphere. This sea water‐dominated hydrothermal system is enriched in reduced iron and rapidly precipitating carbonates and ferrihydrite in the form of bands, having similarities to “Banded Iron Formation” (BIF). BIF represents archives of Earth's primitive biogeochemistry although the combined abiotic and biotic processes that have likely led to their formation are not fully resolved. Diagenesis and metamorphism have a strong imprint on BIF. Thus, continuous efforts are pursued to identify modern analogues that could help unravel their formation. Although carbonate is not a common feature of BIFs, Ilia system provides an interesting analogue for their depositional processes and potential microbial–mineral associations they may have hosted. It also presents pre‐diagenesis facies association and mineralogy that could bring new clues for unravelling BIF modes of formation and the salient biogeochemical conditions characteristic of their original depositional environment.

New fossil discoveries illustrate the diversity of past terrestrial ecosystems in New Caledonia

“…Fossiliferous layers are distributed throughout the whole series, including in some cuirasses (La Madeleine waterfall). The general pattern of the fossiliferous sites is broadly similar to fossiliferous travertine formations, and a few cases of iron travertine deposits are known in Spain 38 , 39 and in Greece, also on ultramafic formations with ferrihydrite encrusted leaves 40 .

Figure 3 Middle(?)

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Insights on the biomineralisation processes and related diversity of cyanobacterial microflora in thermogenic travertine deposits in Greek hot springs (North‐West Euboea Island)

“…Α common characteristic of the hot springs in North‐West Euboea is the deposition of thermogenic travertine (Kanellopoulos, 2012, 2013; Kanellopoulos et al, 2017, 2019). Previous studies have described their mineralogical composition, mineral chemistry, and their relationship with the chemical composition of the ambient hot water (Kanellopoulos, 2012; Kanellopoulos et al, 2017, 2018, 2019). Also, the great variety of their facies was presented in detail by Kanellopoulos (2012, 2013).…”

Antimony’s Significance as a Critical Metal: The Global Perspective and the Greek Deposits

“…Several metallic mineral phases including sulfides (such as pyrite, arsenopyrite, galena, chalcopyrite, sphalerite, and stibnite), native elements (such as Pb and Ni) and native iron [152], alloys (such as Au ± Cu-Ag), fluorite, and REE-bearing phases were identified as being syngenetically enclosed as clastic grains within the pores of all studied travertine [150]. The northwestern Euboea Island hydrothermal system represents the first documented active terrestrial mineralizing hydrothermal system associated with ore-bearing travertine in Greece [150,151,153].…”