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Iron ore mine remediation in Brazil presents a unique challenge, distinct from remediation efforts of other lateritic deposits, as a result of the lack of soil. These unique rupestrian (rock associated) ecosystems, including a suite of naturally rare and endemic plant species, are associated with the ironstone (canga) outcrops that commonly cap iron ore mines. To reinstate native ecosystems postmining, developing a substrate similar to the iron-rich cap rock is required. These iron-rich duricrusts have evolved by the ongoing dissolution and re-precipitation of iron oxide minerals, continuously forming new iron cements. Biological processes respond to fluctuating redox conditions that occur in micrometre-scale anoxic/reducing conditions within a broadly oxidizing environment. Reductive mineral dissolution is critical to the initiation of iron cycling that contributes to the evolution and stabilization of these duricrusts. Understanding the mechanisms that contributes to natural iron cement formation allows for the development of novel biotechnologies that aim to accelerate these processes. These biotechnologies would offer an environmentally friendly, circumneutral strategy for iron ore mine remediation. Successful remediation programmes would aim to accelerate the formation of iron cements necessary to stabilize crushed materials on hillslopes, restore a functional hydrology and regenerate a substrate similar to an iron-rich duricrust for revegetation using native species. Here, we review recent advances in understanding the biological processes that contribute to canga evolution and their implications for the remediation of iron ore mines in the future.

Novel biotechnologies are required to remediate iron ore mines and address the increasing number of tailings (mine waste) dam collapses worldwide. In this study, we aimed to accelerate iron reduction and oxidation to stabilize an artificial slope. An open-air bioreactor was inoculated with a mixed consortium of microorganisms capable of reducing iron. Fluid from the bioreactor was allowed to overflow onto the artificial slope. Carbon sources from the bioreactor fluid promoted the growth of a surface biofilm within the artificial slope, which naturally aggregated the crushed grains. The biofilms provided an organic framework for the nucleation of iron oxide minerals. Iron-rich biocements stabilized the artificial slope and were significantly more resistant to physical deformation compared with the control experiment. These biotechnologies highlight the potential to develop strategies for mine remediation and waste stabilization by accelerating the biogeochemical cycling of iron.

Wolframite [(Fe,Mn)WO

Supergene enriched iron ore deposits in Brazil are typically blanketed by goethite-cemented breccias that form a protective duricrust known as canga. Moderately hard, well consolidated, permeable and resistant to erosion and chemical weathering, the canga blanket protects the relatively friable iron ore below. The protective canga horizons in the Carajas and Quadrilatero Ferrifero mineral provinces represent some of the longest-lived, continuously exposed land surfaces on Earth, and their formation is essential to supergene iron ore enrichment and preservation. Remarkably, the iron-rich duricrusts that have developed in Brazilian tropical rainforest environments, i.e, Carajas, yield geochronological results that indicate that these ancient erosion-resistant surfaces continue to evolve today. Active biogeochemical iron cycling is essential for the ‘self-healing’ cementation/re-cementation occurring in canga, suggesting that recurrent iron reduction and subsequent oxidation are responsible for canga evolution. Macroscopic biological features in canga including ferruginised plant roots and termite tracks have been linked to the biogeochemical cycling of iron. The ‘organic’ textures in canga can be traced to the microscopic scale, preserving fossilised bacterial cell envelopes and permineralised biofilms. At the canga surface, naturally rare and endemic rupestrian plant species carve out an existence, commonly in the absence of soil. Growth of grasses also promotes metal cycling highlighting that the rhizosphere contributes to canga evolution. The fossilisation of microbial biofilms and rhizosphere horizons consolidates canga, affecting its permeability, limiting water transport and enhancing biogeochemical cycling. The development of canga has been essential for the formation, preservation, and discovery of iron ore deposits, and its restoration will ultimately be required for mined land remediation of these unique ecosystems.

As society transitions to low-carbon, renewable energy resources, the demand for metals and minerals is set to increase. Massive quantities of base metals and mineral materials (for example, silica and concrete) along with smaller quantities of precious metals will be required for the construction of wind turbines, solar panels and battery storage facilities to meet the demands of the ‘Electric Planet’ of the future. Harnessing microbe-mineral-metal interactions may offer many opportunities to improve some mining practises and support the long-term sustainability of mining. As easily exploitable, high-grade deposits are becoming increasingly depleted there is a need for new technologies to improve exploration and mining strategies. Microorganisms are ubiquitous and diverse, surviving in almost all environments in the Earth's crust and recent advances in molecular techniques have enabled scientists to study these communities is extraordinary detail. Microorganisms also interact directly with their environment; both responding to and changing the environment around them. These responses and their influences on the surrounding environment are preserved within their genome (a complete set of the DNA of the microorganism). Here, we discuss using state-of-the-art sequencing techniques to identify key microbial genes that have been demonstrated to correlate with metal concentrations. These genetic-based bioindicators may provide additional tools to guide and improve the success rate of mineral exploration programmes. Advances in molecular techniques will also improve existing biohydrometallurgical techniques and expand the commodity range for which biohydrometallurgy are currently economically viable. Finally, microorganisms may be used in a number of strategies for mine remediation; specifically, we review in detail microbially accelerated carbon capture and storage strategies and mine waste stabilisation.

Iron-rich duricrusts (canga) that cap iron ore deposits in Brazil evolve by the redox-based cycling of iron oxide minerals, continually forming new iron-rich cements that effectively prevent the physical erosion of these landforms. This thesis aimed to understand the biological influences on the geochemical cycling of iron within canga, to determine if these processes could be used to accelerate iron cement formation, for example, in a post-mining context. Re-cementing waste material (canga) from iron ore mining may provide an environmentally-friendly solution for iron ore mine remediation in Brazil and the restoration of a rare ecosystem associated with canga outcrops. Field emission scanning electron microscopy revealed an array of microorganisms fossilised within canga, highlighting the innate role of microorganisms in the evolution of canga horizons. Nanoscale elemental mapping of these microfossils revealed a role for aluminium in the long-term preservation of organic and inorganic biosignatures. A suite of high-resolution analytical techniques were correlated to understand the fossilisation processes that contribute to biosignature preservation in near-neutral iron-rich environments. Further investigation of the microfossils in canga, revealed the complex weathering patterns induced by microorganisms. Grains within canga could be completely weathered and texturally replaced by microfossils; only remnants of the grain’s shape and traces of most immobile elements (for example, thorium) were left behind. The minor and trace element proportions of the canga cements revealed the microscale redistribution of titanium, likely to be driven by microorganisms, and the long-range redistribution of phosphorus, both preserved within these iron-rich duricrusts. Aeolian phosphorus from Africa appears to provide an external phosphorus source for the canga-associated ecosystems.As part of fieldwork in the montane regions of Northern Brazil, a lake-edge microbial ecosystem where canga cements appeared to form relatively rapidly was characterised. Examining these rocks, fossilised biofilms were found to play an intricate role in the formation of the meniscus-type cements within canga. Without the organic framework provided by microbial biofilms, and connecting grains together, the iron oxide minerals would simply have enlarged the existing grains without making cements. Armed with new information revealing the microbiome in canga plays an important role in the dissolution and the re-precipitation of iron oxides, these processes were applied to accelerate the reformation of iron-rich crusts in the laboratory. Microbially-mediated iron reduction was promoted within an open-air bioreactor and the iron-rich solutions were allowed to flow over crushed canga on a slope of approximately 10°. Astonishingly, within 6 months the top 3 – 5 cm of the experiment had been stabilised by fossilisation of the microbial biofilms that naturally aggregate materials. This experiment is a platform for field-scale trial aiming to progressively stabilise the slope of an expended mine site. Understanding and harnessing microbial processes that contribute to iron cement formation in canga offers an alternative remediation strategy for iron ore mines in Brazil.

Mineralogical and whole rock geochemical analyses for 60 elements on 31 samples of hard ferruginous crust (canga) provide insights into the evolution of the lateritic profile developed on itabirite. Canga can form in two environments: in situ canga that typically caps itabirite and transported canga that covers country rock. Both have similar mineralogical and chemical compositions. Detrital haematite and rare quartz inherited from the itabirite and iron ore comprise the matrix of canga, cemented by goethite, minor gibbsite, and rare manganese oxides and secondary phosphates. Fe2O3 represents more than 91% of its chemical composition and the concentrations of trace elements are low, generally less than 50 ppm. A comparison of the chemical weathering of dolomitic itabirite against the quartz itabirite shows that, although weathering processes are less effective in the former, the geochemical trends of major and trace elements are similar. Negative Ce anomalies (Ce/Ce* = 0.8) and U/Th ratios lower than 1.5 suggest that saprolite formation occurred under slightly anoxic and mildly acidic conditions, allowing rare earth elements (REEs) to remain in the saprolite and also the formation of secondary Al phosphates, instead of Fe phosphates. These conditions became more aggressive during the canga formation process, resulting in further removal of trace elements from the system. The canga formation (pedogenesis) and the chemical weathering of the itabirite (saprolite formation) are independent, but interrelated processes that have been occurring since the Palaeocene.

The surface crust that caps highly weathered banded iron formations (BIFs) supports a unique ecosystem that is a post-mining restoration priority in iron ore areas. Geochemical evidence indicates that biological processes drive the dissolution of iron oxide minerals and contribute to the ongoing evolution of this duricrust. However, limited information is available on present-day biogeochemical processes in these systems, particularly those that contribute to the precipitation of iron oxides and, thus, the cementation and stabilization of duricrusts. Freshly formed iron precipitates in water bodies perched on cangas in Karijini National Park, Western Australia, were sampled for microscopic and molecular analyses to understand currently active microbial contributions to iron precipitation in these areas. Microscopy revealed sheaths and stalks associated with iron-oxidizing bacteria. The iron-oxidizing lineages Sphaerotilus, Sideroxydans, and Pedomicrobium were identified in various samples and Leptothrix was common in four out of five samples. The iron-reducing bacteria Anaeromyxobacter dehalogens and Geobacter lovleyi were identified in the same four samples, with various heterotrophs and diverse cyanobacteria. Given this arid, deeply weathered environment, the driver of contemporary iron cycling in Karijini National Park appears to be iron-reducing bacteria, which may exist in anaerobic niches through associations with aerobic heterotrophs. Overall oxidizing conditions and Leptothrix iron-oxidizers contribute to net iron oxide precipitation in our sampes, rather than a closed biogeochemical cycle, which would result in net iron oxide dissolution as has been suggested for canga caves in Brazil. Enhancements in microbial iron oxide dissolution and subsequent reprecipitation have potential as a surface-crust-ecosystem remediation strategy at mine sites.

Bioremediation of degraded lands using biological soil crusts (i.e., biocrusts; complex association between soil, microorganisms, and extracellular polymeric substances) is an emerging biotechnological strategy to control surface erosion. Here, we examined the use of biocrusts for the stabilisation of iron ore mine waste. Simulating naturally occurring biomineralisation of microbial mats on ferricrete, we promoted the ferrugination of biocrusts on crushed iron-rich waste material, aiming to form iron biocement. The biocrusts on our 50-cm experimental slopes (comprised primarily of cyanobacteria, green algae, and fungi) aggregated crushed ferricrete fragments through the secretion of extracellular polymeric substances. Provided with an external source of ferrous iron, the biocrusts also acted as organic scaffolds for the precipitation of iron oxides. The mineralisation of the biocrusts led to the formation of biocemented aggregates that mechanically stabilised the crushed iron-rich waste material. The biocrusts, with and without biomineralisation, enhanced hydraulic conductivity and minimised erosion. The addition of ferrous iron without an organic framework resulted in a powdery coating of individual fragments, rather than consolidation of the substrate. Our findings demonstrate the potential of synthetic biocement as a long-term stabilisation strategy for waste rock stockpiles, engineered slopes, and mine remediation requiring the reformation of iron-rich duricrust.

Understanding the natural microbiological mechanisms that promote iron cycling in iron ore mine environments may provide novel tools for the remediation of the fragile, iron-rich duricrust ecosystems associated with these environments as well as provide a solution for the stabilisation of hillslopes and tailings (waste) dams. A diverse array of microfossils is frequently identified throughout metrescale duricrusts (canga; >50 wt.% Fe) that cap iron ore deposits in Brazil, shedding light on the intimate role of microorganisms in the evolution of these crusts. Nanoscale secondary ion mass spectrometry revealed that carbon and nitrogen biosignatures are occasionally preserved, and typically associated with the cell envelope structures of microfossils. The microfossils are 1-5 mm in length, with filamentous and rod-shaped morphologies commonly preserved1,2. When examined using backscatter electron scanning electron microscopy, canga shows a complex microstructure from repeated dissolution and re-precipitation of iron oxide minerals. Geochronology3, geochemistry4 and microbiology5 provide insights into the past and present-day role of microorganisms in the evolution of canga. These dynamic biogeochemical processes in canga contribute to the continuous formation of new iron cements, preserving some of world's longest-lived continuously exposed surfaces. Harnessing and accelerating the biogeochemical cycling of iron may contribute to the development of novel technologies for mine remediation and waste stabilisation.

Demonstrating the biogenicity of presumptive microfossils in the geological record often requires supporting chemical signatures, including isotopic signatures. Understanding the mechanisms that promote the preservation of microbial biosignatures associated with microfossils is fundamental to unravelling the palaeomicrobiological history of the material. Organomineralization of microorganisms is likely to represent the first stages of microbial fossilisation and has been hypothesised to prevent the autolytic degradation of microbial cell envelope structures. In the present study, two distinct fossilisation textures (permineralised microfossils and iron oxide encrusted cell envelopes) identified throughout iron-rich rock samples were analysed using nanoscale secondary ion mass spectrometry (NanoSIMS). In this system, aluminium is enriched around the permineralised microfossils, while iron is enriched within the intracellularly, within distinct cell envelopes. Remarkably, while cell wall structures are indicated, carbon and nitrogen biosignatures are not preserved with permineralised microfossils. Therefore, the enrichment of aluminium, delineating these microfossils appears to have been critical to their structural preservation in this iron-rich environment. In contrast, NanoSIMS analysis of mineral encrusted cell envelopes reveals that preserved carbon and nitrogen biosignatures are associated with the cell envelope structures of these microfossils. Interestingly, iron is depleted in regions where carbon and nitrogen are preserved. In contrast aluminium appears to be slightly enriched in regions associated with remnant cell envelope structures. The correlation of aluminium with carbon and nitrogen biosignatures suggests the complexation of aluminium with preserved cell envelope structures before or immediately after cell death may have inactivated autolytic activity preventing the rapid breakdown of these organic, macromolecular structures. Combined, these results highlight that aluminium may play an important role in the preservation of microorganisms within the rock record. Keywords: Aluminium, Microfossils, Biosignatures, NanoSIMS, Organomineralisation

The evolution of iron-rich duricrusts (canga) that cap weathered banded iron formations (BIFs) in the Serra Sul de Carajas, northern Brazil, has been essential to armour these landscapes, allowing for the enrichment and preservation of world class iron ore deposits. Bulk chemical data highlight an incorporation of exogenous materials into secondary cements, including aluminium, phosphorus, silicon, titanium, vanadium and zirconium. The incorporation of these materials within the canga cements highlights the continuous formation of secondary cements within canga, which entrap these materials. Microfossils identified throughout the canga profiles indicates microorganisms are likely to have contributed to mineral weathering, metal mobility and biomineralisation. X-ray fluorescence microscopy (XFM) analysis of canga subsamples revealed titanium mobility in close proximity to preserved microfossils. Microbially accelerated mineral weathering appears to place titanium into solution, which reprecipitates as microcrystalline anatase within the goethitic cements that forms rims around some grains. These results demonstrate the mobility of titanium within duricrusts, which is likely to be influenced by organic acids. Canga evolution was investigated using X-ray absorption near edge structure (XANES) analysis combined with bulk chemical extractions. These experiments demonstrate that hematite (martite) fragments are enriched in ferrous iron, which is released during acid-induced weathering. This ferrous iron is likely to be maintained in microscale magnetite inclusions within the martite. These data support the nonredox transition of magnetite to martite (likely via a ferrous iron-deficient magnetite (for example, kenomagnetite)). The ferrous iron released during the weathering of magnetite-bearing rocks provides an iron source for the generation of new cements within canga. These insights have implications for choosing starting materials for the re-cementation of iron-rich duricrusts after the completion of iron ore mining in Brazil.

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 (CH2) 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.

Iron-rich duricrusts (canga) that blanket iron ore deposits in tropical regions in Brazil have a complex formation and evolution. The direct role of microorganisms in the biogeochemical cycling of iron and remarkably aluminium, which is critical for the preservation of these duricrusts, is becoming more apparent. In this study we combine nanoscale secondary ion mass spectrometry (NanoSIMS) with Raman spectroscopy and scanning electron microscopy to distinguish highly weathered and recycled grains, portions of which have been metasomatised by iron in solution, from new mineral precipitates within canga. Microbially-accelerated weathering of grains within canga appears to promote iron, aluminium and titanium oxide mineral dissolution. Anatase forms at the boundaries of rutile grains, highlighting titanium dissolution and re-precipitation at the mineral-scale. Titanium is absent from the authigenic cements in canga. Aqueous ferrous iron, which is sensitive to changes in the oxidation potential of the solution, readily oxidises and re-precipitates in the pore spaces in proximity to preserved microorganisms, leaving aluminium as the most abundant cation in solution. Changes in solution pH and/or organic chemistry causes aluminium to precipitate from solution as gibbsite that infills pore spaces as the final geological texture. Evidence for microbially-promoted weathering includes the fossilisation of microorganisms along grain boundaries, in which aluminium, titanium, chromium and iron are depleted. Immobile elements including titanium, chromium and thorium, associated with rounded fossilised biofilms also indicate that microorganisms have completely weathered grains and become fossilised in place, texturally replacing the grain in the process. Microorganisms also play a role in iron and aluminium precipitation, providing sites for mineral nucleation. These results highlight the microscale changes in oxidation potential, pH and organic chemistry throughout canga that are likely to be influenced by the microbiome and associated flora. Understanding the role of microorganisms in iron biogeochemical cycling will contribute to the accelerated re-cementation of iron-rich duricrusts after the completion of iron ore mining, providing a substrate for revegetation using native plant species. This study also sheds light on direct (microfossils) and indirect (biologically-influenced element mobility) biosignatures in iron-rich geological materials.

Goethite-cemented duricrusts, also known as canga, commonly occur as a capping rock protecting underlying iron ore deposits. The processes that govern canga formation are still unclear but include recurrent partial dissolution and recrystallisation of goethite through biogeochemical cycling of iron, hypothesised to be catalysed by plants and bacteria. In the present study, the effect of plant exudates on mobilisation of iron in canga was examined using model plants grown on crushed canga in RHIZOtest devices, which separate roots from substrate by a semi-permeable membrane. Moderate plant-induced acidification of the canga was detected, however the primary driver of mineral dissolution was the synergistic effect of reductive and ligand-promoted dissolution, identified by an increase in organic acids concentration and the presence of low concentrations of free ferrous iron. Whilst organic acids exudation lasted, iron cations were stabilised in solution; once the organic acids were degraded by microorganisms, the free cations precipitated as iron oxy-hydroxides. Mineralogical analysis and high-resolution microscopy confirmed our hypothesis that plants that grow in this iron-rich substrate contribute to iron dissolution indirectly (e.g., during phosphate solubilisation), and that the resulting surplus iron not taken up by the plants is redeposited, promoting the cementation of the residual minerals. Understanding the contribution of plants to the iron cycling in canga is crucial when formulating post-mining rehabilitation strategies for iron ore sites.

Microbial biofilms growing in iron-rich seeps surrounding Lake Violão, Carajás, Brazil serve as a superb natural system to study the role of iron cycling in producing secondary iron cements. These seeps flow across iron duricrusts (referred to as canga in Brazil) into hydraulically restricted lakes in northern Brazil. Canga caps all of the iron ore deposits in Brazil, protecting them from being destroyed by erosion in this active weathering environment. Biofilm samples collected from these seeps demonstrated heightened biogeochemical iron cycling, contributing to the relatively rapid, seasonal formation of iron-rich cements. The seeps support iron-oxidising lineages including Sideroxydans, Gallionella, and an Azoarcus species revealed by 16S rRNA gene sequencing. In contrast, a low relative abundance of putative iron reducers; for example, Geobacter species (