Fluid-Rock Reactions and Hydrologic Drivers of Iron Cycling in Former Redbed Sandstones
Tong Guo1, Grant Ferguson1,2, Coleman Hiett3, Peter Reiners3, and Jennifer McIntosh1,2
1Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA,
2Department of Civil, Geological and Environmental Engineering, University of Saskatchewan, Saskatoon, SK, 3Department of Geosciences, University of Arizona, Tucson, AZ, USA
Iron cycling, often enhanced by microbial activity, is a common process from the land-surface to the bottom of the meters- to kilometers-deep Critical Zone, leaving paleofluid flow imprinted in the rock record. Bleaching of redbed sandstones by reduced fluids and iron oxide precipitation in joints with varying geometry is widespread across the Colorado Plateau. Questions remain about the fluids involved, hydrologic drivers, and timing of iron bleaching/mineralization events. We hypothesize that topographically-driven, hydrocarbon-bearing reduced fluids dissolved hematite in the Navajo Sandstone, mobilizing Fe2+ downgradient in regional aquifers. Localized joints acted as conduits for reduced fluids upward migration and mixing with shallow oxic meteoric waters, precipitating iron oxides. A 2D hydrogeochemical model (PFLOTRAN) was used to compare with field observations to investigate the mechanisms, timescales, extent of fluid mixing and fluid-rock reactions in the Navajo Sandstone near gas fields in the Escalante Anticline.
Model results indicate reduced, CH4- and CO2-bearing groundwater can dissolve hematite (1% volume) within 600,000 years in relatively high permeability zones, lower permeability zones retain hematite for up to 5 Ma. Calcite and illite fully dissolve, quartz precipitates, and K-feldspar and kaolinite decrease but remain in the bleached sandstone, consistent with XRD results. Mixing of Fe2+ saturated, reduced deep groundwater, migrating upwards along joints, with oxic shallow groundwaters can precipitate goethite in pipe-like structured downgradient in the bleached aquifer, as observed in the field, within 2 Ma. These results consistent with previous (U-Th)/He dating of goethite-dominated iron oxides and help constrain hydrologic and geochemical processes driving iron cycling and concretion formation.