Silica and iron mobilization, cave development and landscape evolution in iron formations in Brazil

Abstract

Abstract Iron formations display some of the oldest and most enigmatic landforms on Earth, presently restricted to few areas of the planet. The original rock, the Banded Iron Formation (BIF) was formed in the Proterozoic and has been continuously weathered through a complex interplay of chemical and biological processes involving the massive mobilization of the main elements silica and iron, resulting in silica depleted (but friable) high-grade iron bodies and a Fe-rich surficial duricrust known as canga. This study presents new data and includes a comprehensive review of the existing literature, incorporating extensive data from unpublished reports and new findings. It aims to quantitatively analyze the morphology and development of porosity in iron formations both at micro and macro scales, assessing the geobiological mechanisms responsible for iron and silica mobilization and their role in the dynamics of this ever-evolving landscape. Silica leaching is the initial process of porosity generation at depth in the phreatic zone and results in a productive aquifer that contributes to the removal of solute and the generation of initial low-density zones. The development of numerous pores and voids is favored by the permeability contrast at the shallow contact zone between the canga and the weathered ore. Caves tend to occur at the base of scarps at the limit of plateaus and ridges, and tend to scale down in size away from the borders. The landscape evolves through the retreat of scarps, mostly through the collapse of cave passages. Caves exhibit a characteristic morphology that intercalates larger rooms and smaller connecting passages, suggesting that macropores started initially as isolated voids and occur either entirely in canga or in the weathered BIF, but commonly are associated with the contact between these rocks. Caves in the Amazonian Carajás region display the largest dimensional values, regardless of bedrock context. Longer caves show a larger number of connections between rooms, suggesting that the coalescence of rooms mostly through slope interflow processes is a key player in fostering the development of caves. Fractal Dimension (FD) calculated for the larger caves demonstrates that there is no relationship between lithology and FD, although values of FD vary between regions, with Carajás displaying the highest ones.

Geomicrobiological processes play a major role in the mobilization of iron through Fe(III) reducing bacteria. In particular, it promotes the long-term stabilization of voids through the generation of a hardened surface that protects the walls of caves and voids from collapse. The interplay of chemical and bacterial processes allows for the generation of a highly integrated network of pores and caves that represent outlets for the removal of silica and mobilization/concentration of iron. These voids are the result of geochemical leaching and evolve synchronously with the landscape, mostly at scarp level. Such processes connect isolated pores, promote evacuation of solutes and allow for the slow lateral degradation of the landscape. This geologically slow process is now heavily affected by mining operations, which reinforces the need for searching for mitigation and/or regeneration pathways in order to preserve this unique geo-biological ancient landscape.