Περίληψη:
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.
Συγγραφείς:
Kanellopoulos, Christos
Thomas, Camille
Xirokostas, Nikolaos and
Ariztegui, Daniel
