Dating circulations of hydrothermal fluids in the crystalline basements of unconformity-related metallic deposits using in situ Rb/Sr geochronology: proof of concept

Abstract. The use of in situ Rb-Sr geochronology has boomed in recent years following its implementation using LA-ICP-QQQ-MS technology, which enables fast, in situ analyses at the micron scale on selected minerals. The Rb-Sr geochronometer applied to micas is now commonly used to date the crystallization or cooling of metamorphic and magmatic rocks, based on the assumptions of a closed isotopic system after passing the closure temperature and of a homogeneous Sr isotopic composition at the time of crystallization. In situ Rb-Sr geochronology applied to micas and related alteration products in geological contexts involving hydrothermal fluid circulation affecting micas after crystallization could provide a new way to decipher the timing and duration of fluid circulation in various settings such as mountain belts or sedimentary basins. The behavior and applicability of the Rb-Sr system in such contexts are, however, poorly understood, as the system may be partially reopened with differential redistribution of Rb and Sr at the grain scale. To test this hypothesis, we selected a case study related to unconformity-related U deposits from the Athabasca Basin (Canada), which formed through intense hydrothermal fluid circulation at the interface between crystalline basement and siliciclastic sedimentary rocks and represent archetypes of unconformity-related metallic deposits. Muscovite grains from metamorphic and magmatic rocks were targeted across a range of alteration states, from hydrothermally unaltered to strongly altered domains. We focused on a specific hydrothermal alteration linked to the formation of hydrothermal illite and sudoite at the expense of metamorphic or magmatic minerals. In unaltered zones, muscovite displayed variable but high Rb/Sr ratios, whereas the 87Sr/86Sr intercepts derived from Rb-Sr regressions were scattered and were not interpreted as meaningful initial isotopic compositions. The resulting ages ranged from ca. 1870 to ca. 1720 Ma and were consistent with the geological context. In distal-to-proximal alteration halos of U deposits, muscovite and related alteration products yielded lower 87Rb/86Sr ratios and highly variable regression intercepts. The mean age calculated across the different samples and investigated sites clustered around ~1640 Ma, a value previously obtained by Ar-Ar geochronology on illite and U-Pb geochronology on other hydrothermal phases and proposed to correspond to a major hydrothermal event linked to a geodynamic reorganization affecting the Canadian Shield at the circum-Laurentian scale. The ~1640 Ma age is geologically meaningful in the studied context and is interpreted as reflecting partial, micrometric-scale resetting of the Rb-Sr system in muscovite during this hydrothermal event. The wide range of regression intercept values commonly observed in disturbed Rb–Sr systems is interpreted as an apparent result of open-system behavior, reflecting partial system reopening and non-conservative redistribution of Rb and Sr at the grain scale, rather than as a physically meaningful initial isotopic composition. These results demonstrate that detailed analysis of Rb-Sr system perturbations in altered muscovite and related alteration products can constrain the timing of ancient hydrothermal activity and the spatial dynamics of fluid-rock interaction. This approach provides a valuable complement to conventional fluid-tracing methods and opens new perspectives for reconstructing paleo-hydrothermal systems in ancient basement terrains.