Evidence of biogeochemical processes in iron duricrust formation

Abstract

Canga is a moderately hard iron-rich duricrust primarily composed of goethite as a result of the weathering of banded iron formations. Canga duricrusts lack a well-developed soil profile and consequently form an innate association with rupestrian plants that may become ferruginised, contributing to canga possessing macroscopic biological features. Examination of polished canga using a field emission scanning electron microscope (FE-SEM) revealed the biological textures associated with canga extended to the sub-millimetre scale in petrographic sections and polished blocks. Laminae that formed by abiotic processes and regions where goethite cements were formed in association with microorganisms were observed in canga. Biological cycling of iron within canga has resulted in two distinct forms of microbial fossilisation: permineralisation of multispecies biofilms and mineralisation of cell envelopes. Goethite permineralised biofilms frequently formed around goethite-rich kaolinite grains in close proximity to goethite bands and were composed of micrometre-scale rod-shaped, cocci and filamentous microfossils. In contrast, the cell envelopes immobilised by authigenic iron oxides were primarily of rod-shaped microorganisms, were not permineralised and occurred in pore spaces within canga. Complete mineralisation of intact rod-shaped casts and the absence of permineralisation suggested mineralised cell envelopes may represent fossilised iron-oxidising bacteria in the canga ecosystem. Replication of these iron-oxidising bacteria appeared to infill the porous regions within canga. Synchrotron-based Fourier transform infrared (FTIR) microspectroscopy demonstrated that organic biomarkers were poorly preserved with only weak bands indicative of aliphatic methylene (CH 2) associated with permineralised microbial biofilms. High resolution imaging of microbial fossils in canga that had been etched with oxalic acid supported the poor preservation of organic biomarkers within canga, indicating mineralogical replacement of organic biomarkers.

A Combined (U‐Th)/He and Cosmogenic 3He Record of Landscape Armoring by Biogeochemical Iron Cycling

“…Pure goethite masses commonly show evidence of several generations intimately intergrown (Figures c and d); goethite masses may also contain small amounts of supergene gibbsite and rarely quartz. Goethite often shows evidence of ferruginized tree roots and bacterial fossils, attesting to the strong role of the biota in iron cementation (Levett et al, ; Monteiro et al, ).…”

“…The transformation from saprolite to canga is both chemical (quartz and carbonate dissolution and leaching) and mechanical (collapse and translocation associated with mass losses during silica and carbonate dissolution) (Figure ). It is also strongly influenced by biological processes (Levett et al, ; Monteiro et al, ). The most important chemical processes are the dissolution and leaching of silica, carbonates, and minor sulfides, and the reductive dissolution of hematite and magnetite with subsequent precipitation of goethite.…”

“…For example, exposure ages for goethite cements are either equal to or less than their precipitation ages (Figure ), suggesting that most goethite cementation occurs in the subsurface, where the cements reside before slowly ascending to the surface. The biogenic processes interpreted to control the self‐healing properties of cangas (Levett et al, ; Monteiro et al, ) may occur both at the surface and the subsurface, but a key ingredient in goethite cementation is reduced and acidic water. Since cangas are very porous and water readily infiltrates into a network of tubules, vugs, and fractures, it is likely that this infiltrating water carries organic acids leached from the surface, accumulates in subsurface void spaces, and establishes a reducing environment that can dissolve iron from hematite ± magnetite ± goethite (Figure ; Table ).…”

Biogeochemical Cycling of Silver in Acidic, Weathering Environments

“…Crevices within gossan allows groundwater to descend and provides ideal microenvironments for microbial attachment and growth [53][54][55]. With regard to iron biogeochemical cycling, microorganisms are known to contribute to the structure of terrace iron formations as well as other ferruginous duricrust such as canga [43,56,57]. Circular and oval structures within gossan ( Figure 3) were interpreted as microbial microfossils based on the mode of biomineralization and cell wall preservation observed from the microbial enrichments ( Figure 4A-C).…”

The geomicrobiology of mining environments

“…Geochemical and microbial fossil evidence suggests that biological cycling of iron has contributed to the evolution of the duricrusts throughout geologic history, particularly the dissolution and reprecipitation of goethite 27,28 ; thus, potentially, present-day biological iron cycling could be harnessed to 're-form' this duricrust on a . Building on insights from field work characterising the structure and function of microbial communities in bauxite residues before, during, and after remediation, our research group has now developed microbially driven approaches for pH neutralisation in bauxite residue that will enable remediation of both existing and future alkaline tailings and wastewater streams 29,30 .…”