Abstract

Iron-oxide coloration and deposits in sandstone are significant indicators of the mobility of solutes (Fe2+ and O2) in groundwater, mainly controlled by host-rock porosity and permeability. We describe the occurrence and geometry of different types of iron-oxide deposits developed within the vadose zone along faults affecting poorly lithified, quartz-dominated, heterolithic sands in the Paraíba Basin, NE Brazil. The development of highly permeable damage zones (100–102 Darcy) and low-permeability fault-core–mixed zones (10–3–101 Darcy) promotes the physical mixing of Fe2+-rich waters and oxygenated groundwater. This arrangement favors iron-oxide precipitation as meter-scale sand impregnations, centimeter- to decimeter-scale concretions, and well-cemented decimeter- to meter-thick mineral masses. The formation of hydraulically isolated compartments along hard-linked strike-slip faults promotes: (1) the development of Liesegang bands in a reaction zone dominated by pore-water molecular diffusion of O2 into Fe2+-rich stagnant water, and (2) the precipitation of iron-oxide impregnations and concretions in the fault-core–mixed zone boundaries, likely by O2 diffusion in flowing Fe2+-rich waters. Late-stage fault reactivation provides preferential pathways for the circulation of gravity-driven reducing fluids, resulting in localized dissolution of iron and bleaching along fractures and iron remobilization. These relationships reveal the roles of tectonic activity and near-surface sandstone diagenesis in determining preferential hydraulic pathways for the physicochemical interaction between oxygenated groundwater and iron-rich fluids. Structural setting, fault-zone architecture, and related grain-size–permeability structures determine the dominant mode of solution interaction, leading to the formation of iron-oxide Liesegang bands where O2 diffuses into stagnant Fe2+-rich water, and concretions when diffusion is complemented by Fe2+ advective flow.

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