Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations

Research Article| April 01, 2013 Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations Birger Rasmussen; Birger Rasmussen * 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia *E-mail: B.Rasmussen@curtin.edu.au. Search for other works by this author on: GSW Google Scholar Daniela B. Meier; Daniela B. Meier 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia †Current address: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Search for other works by this author on: GSW Google Scholar Bryan Krapež; Bryan Krapež 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia Search for other works by this author on: GSW Google Scholar Janet R. Muhling Janet R. Muhling 2Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (4): 435–438. https://doi.org/10.1130/G33828.1 Article history received: 06 Jul 2012 rev-recd: 16 Oct 2012 accepted: 24 Oct 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Birger Rasmussen, Daniela B. Meier, Bryan Krapež, Janet R. Muhling; Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations. Geology 2013;; 41 (4): 435–438. doi: https://doi.org/10.1130/G33828.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF where they define sedimentary laminations, implying a depositional origin. We suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor. The microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. The microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular-rich lamina overlain by amorphous mud with dispersed microgranules. The lamina sets collectively define plane-lamination structure, probably of the lower flow regime. The microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets. Episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in BIF. We propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.