Diagenetic and hydrothermal fluid alteration of ancient sedimentary rocks creates visible ferrous/ferric redox interfaces attributable to changes in iron oxide and phyllosilicate mineralogy. Redox interfaces control aqueous/ solid phase partitioning of many metals such as Cr, U, As, and Mn. However, the long-term implications of ancient redox interfaces for the mobility of metals in modern groundwater remains uncertain. We investigated redox interfaces of lower Permian sediments in contact with Precambrian basement rock to understand how red bed mineralogy impacts metal reactivity during hydrothermal events. The proximal Cutler Formation lies at the mouth of Unaweep Canyon in western Colorado and hosts green, reduced, primarily coarse-grained clastic sediments within red beds while the more distal portion of the proximal Cutler hosts bleached layers within red beds. We hypothesized that color variations result from grain-size variations that regulated flow of reducing fluids during a Tertiary hydrothermal event. Powder X-ray diffraction (XRD) revealed the presence of hematite and chlorite in both red and green sediments. Pervasive hematite suggests sediments were hematite-rich before hydrothermal alteration. Abundant mixed layer clays such as illite-smectite, chlorite-smectite, and lesser chlorite-illite dominate clay fraction mineralogy. Previous studies indicate the presence of kaolinite; however, it is absent in our samples. Optical microscopy and electron microbeam analyses will be performed to identify metal-bearing minerals, especially in association with chloritization. Furthermore, quantitative comparison of red vs. green particle size and surface area will be performed via laser particle size analysis and BET analysis, respectively. Experiments will evaluate iron redox state, reactivity of reduced/oxidized sediments towards metal sorption and reduction, and reactivity of mixed layer clays. Ultimately, understanding the reactivity of redox interfaces in ancient sedimentary rocks will help predict the behavior of redox-active metals such as U(VI) and Cr(VI) and the long-term persistence of sedimentary rock reducing capacity for metal immobilization.

Redox interfaces of the proximal Permian Cutler Formation, western Colorado: Implications for metal reactivity

2019 Spring National Meeting

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