Titanium mobility preserved in association with microfossils in an iron-rich duricrust capping an iron ore deposit

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.

Automated summary

The evolution of iron-rich duricrusts in the Serra Sul de Carajas in Brazil plays a key role in enriching and preserving world-class iron ore deposits. Research shows that various exogenous materials are incorporated into secondary cements within the duricrusts, and microorganisms likely contribute to mineral weathering and biomineralization. The mobility of titanium within the duricrusts and the release of ferrous iron during weathering of magnetite-bearing rocks are also important findings.