A record of paleofluid circulation in faults revealed by hematite (U-Th)/He and apatite fission-track dating: an example from Gower Peninsula fault fissures, Wales

Fault-related fissures on the Gower Peninsula, Wales, preserve evidence of faulting, hematite-calcite mineralization, sediment infill, and paleofluid flow. We combine hematite (U-Th)/He (He) dating with sandstone apatite fission-track (AFT) and apatite and zircon (U-Th)/He (He) thermochronology to constrain the timing of fluid circulation in these structures. Hematite He dates from 141.0 ± 5.1 Ma to 119.9 ± 5.0 Ma overlap with a 131.4 ± 20.1 Ma sandstone infill AFT date. Individual zircon He dates are ~402-260 Ma, reflecting source material erosion, and imply a maximum late Permian infill depositional age. Burial history reconstruction reveals modern exposures were not reheated to temperatures sufficient to reset the AFT or hematite He systems in the Triassic-Early Cretaceous, and thus these dates cannot reflect cooling due to erosion alone. Hot fluids circulating through fissures in the Early Cretaceous reset the AFT system. Hematite was either also reset by fluids or precipitated from these fluids. Similar hematite He dates from fault-related mineralization in south Glamorgan (Wales), and Cumbria (England) imply concomitant regional hot groundwater flow along faults. Hydrothermal fluid circulation, coeval with North Atlantic rifting, occurred in these higher permeability fissures and fault veins long after they initially formed.