Hematite (U-Th)/He and Apatite Fission-track Dating Constrain Paleofluid Circulation in Faults: An Example from Gower Peninsula Fissure Fills, Wales

Abstract:

Hematite-calcite fissure fills on the Gower Peninsula, Wales, preserve evidence of faulting, mineralization, and multiple fluid flow events. Fissures are associated with dilational strike-slip faults in early Carboniferous limestone and contain locally brecciated hematite, calcite, and red sediment. Hematite is macroscopically botryoidal, but lobes comprise an intricate aggregate of 0.15-0.5 μm-thick plates with high aspect ratios. Prior work suggests mineralization occurred in either late Carboniferous or late Triassic time. We combine hematite (U-Th)/He dating with apatite (U-Th)/He, apatite fission-track (AFT), and zircon (U-Th)/He thermochronology of fill materials to evaluate the timing and thermal effects of fluid circulation in these structures. Hematite He data from four fissures yield reproducible dates of 141.0 ± 5.1 Ma to 119.9 ± 5.0 Ma. Individual zircon He dates from a sandstone infill sample are ~402-260 Ma, reflecting erosion of source material, and imply a maximum late Permian depositional age. The sandstone AFT date of 131.4 ± 20.1 Ma overlaps with the hematite He results and the apatite He date is ~50 Ma. Reconstruction of the regional burial history from independent geologic constraints reveals modern exposures were not buried and reheated to temperatures hot enough to reset the AFT or hematite He systems in the Triassic-Early Cretaceous. Thus, these data do not simply record ambient cooling from erosion. Hot fluids (~100-150 °C) circulating through fissures in the Early Cretaceous reset the AFT system. Hematite either formed in the Triassic and was also reset by fluids or formed from these fluids. Similar hematite He dates from fault-related mineralization in adjacent south Glamorgan and Cumbria, England, imply concomitant regional hot fluid effects. Our data document hydrothermal fluid circulation, coeval with opening of the North Atlantic Ocean, in these higher permeability fissures and fault veins long after they initially formed. Results suggest hematite and apatite are susceptible to thermal resetting by hot fluids and that these thermochronology tools can be used to constrain paleofluid flow events in the shallow crust.