A two‐stage, fault‐controlled paleofluid system at the southern termination of the Gypsum Valley salt wall, Paradox Basin, Colorado, USA

This study combines field structural analysis with thin‐section petrography, U‐Pb dating, and strontium, carbon and oxygen isotopic analysis of calcite fracture fills to constrain the evolution of the 2‐5 km scale paleofluid system around the faulted, plunging fold nose comprising the southern termination of the Gypsum Valley salt wall in the Paradox Basin, U.S.A. Brittle deformation in this region began with the formation of a down‐to‐the‐northeast, counter‐regional fault and then progressed into jointing and faulting in a radial pattern, followed by jointing in a concentric pattern. Coupled with increases in fracture abundance toward the faults, multiple stages of mineralization suggest that the faults served as efficient and long‐lived conduits for vertical fluid migration. Although fracture cement textures and calcite colour are variable throughout the area, the distribution of these characteristics does not correlate with fracture orientation, relative age, stratigraphic or structural position. Irrespective of the type of calcite comprising the fracture cements, δ13C values average near −7‰ (VPDB), whereas δ18O values cluster into groups whose averages are roughly 6‰ apart, with the more negative grouping stratigraphically restricted to fracture cements in Jurassic rocks. The stratigraphic segregation of δ18O values suggests the paleofluid system contained two distinct paleofluids, a more recent one comprised of meteoric waters and an older one comprising brine that originated in Pennsylvanian strata. 87Sr/86Sr ratios in fracture‐filling calcite cements indicate that the older fluid underwent fluid‐rock interaction with Permian strata and that this evolved fluid migrated upwards along the faults until the Triassic or Jurassic. Thereafter, fluid migrating along the faults was more meteoric and appears to have migrated downward along the faults, where it interacted with Permian strata. Consistent U‐Pb dates from carbonates precipitated from the older fluid suggest this stage of the paleofluid system was active around 240 Ma. Local burial history models and published temperatures for fracture cements elsewhere in the basin suggest the younger stage of the paleofluid system occurred during the Latest Cretaceous to Oligocene. This study highlights the spatial and temporal complexity of fluid systems in the vicinity of salt structures and emphasises the need to interpret them through careful integration of high resolution stratigraphic and structural data i