Fluid-rock Interaction and Episodic Fluid Flow within the Hurricane Fault-zone

Abstract:

The Hurricane Fault is an active 250-km long, west dipping, Basin and Range bounding normal fault in SW Utah and NW Arizona. Fault rock alteration and mineralization is common in the damage zone along strike, indicating that this structure has influenced past groundwater flow. Multiple Quaternary basaltic centers are located proximal to the fault. This study tests the hypothesis that fault-zone diagenesis is being driven by deeply circulated meteoric groundwater infiltration and associated rock-water interaction that is punctuated by periods of hydrothermal alteration associated with local magmatism. Fault-parallel/oblique fractures and small-offset antithetic and synthetic normal faults have been found within fault-zone rocks. The intensity of fracturing and associated evidence of fluid-rock interaction progressively decreases away from the main fault trace into the footwall. Host rock alteration, hematite mineralized fault surfaces, and calcite and hematite cemented deformation bands and veins are observed. These features are focused in 1 – 2 m wide zones of fracturing with densities of 6 – 18 m^-1 located within the footwall damage zone. Host rock alteration in the form of both “bleaching” and oxidation along fractures provides evidence for past redox reactions. Mineralization in deformation bands suggests that some fluid flow and diagenesis was penecontemporaneous with deformation. Laminations and cross-cutting relationships in veins indicate periodic mineralization that could be controlled by episodic fluid flow, or fracturing and degassing leading to calcite precipitation. Stable isotopic results from calcite veins show δ¹³C_PDB values of -7 to 3 ‰ and δ¹⁸O_PDB values of -19 to -9 ‰. Carbon stable isotope ratios suggest multiple carbon sources such as marine carbonates, organic sedimentary rocks, and mantle derived CO₂. Temperature differences in paleofluids and associated fluid-rock interaction may explain the observed range in δ¹⁸O values. Fluid inclusion microthermometry is combined with the δ¹⁸O of calcite to constrain the temperature and the oxygen isotopic composition of the paleofluids. The Hurricane Fault-zone exhibits fault-rock diagenesis focused in densely fractured, fault-parallel zones within the damage zone that provide insight into paleofluid chemistry and temperatures.