| 20 Jan 2026
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.
There is a lot to unpack in this contribution and I commend the authors on a comprehensive synthesis of this timely topic. The writing is excellent if somewhat dense in spots. I appreciate the attention to previous literature. The design of the project is thoughtful, and the data is collected using highly suitable methods. Incorporating textures and compositional data to support in situ Rb-Sr is essential and the authors do an overall excellent job of contextualizing the isotopic data. This is absolutely required in a setting like the Athabasca Basin where prolonged high-T to low-T hydrothermal activity has created a wide range of alteration assemblages during basin formation and readjustment. Integrating the in situ Rb-Sr data with models for U deposition is very novel indeed. Also particularly noteworthy is the suggestion, supported by the data, that partly altered white micas underwent preferential liberation of 87Sr* relative to non-radiogenic Sr. This could be a excellent case to follow-up using atom probe measurements to examine the distribution of Rb vs. 87Sr* and 86Sr in the mica lattice.
The only suggestion I can make is that the order of presentation of some important text could be improved. For example we read through all of the results and note that the initial Sr intercepts are impossibly low. I was questioning to myself if this was an analytical bias. It is only in the discussion that the robustness of La Posta secondary material is described which allays some fears of analytical bias. So perhaps it would be good to state more clearly the reproducibility of La Posta age AND initial Sr intercept at the end of the methods section.
There are a few other minor things I’ve highlighted below. Notwithstanding these the manuscript is in very good shape and illuminates some very important processes affecting a world-class U camp.
Line 154: switching here from xxxx Ma to x.xx Ga.
Line 167: please consider italicizing P and T here and elsewhere
Line 336-340: La Posta is listed as a secondary standard. But it is not obvious from the text how well it was reproduced based on NIST610/MicaMg-NP external calibration? I also note that no additional error was added to the final isochron ages and they are quoted as 1σ starting on line 456. This is a bit unconventional, with most studies using in situ Rb-Sr citing 2SE and including additional error reflecting the long-term reproducibility of secondary standards. Is the 1σ required to use the Gaussian Mixing Model? (this looks a lot like Isoplot ‘unmix age’ routine). What happens to this unmixing exercise if you use 2SE rather than 1σ? This gets back to La Posta results. What was the measured error on La Posta in this study? We are only given the Zack & Hogmalm 2016 reported value.
Line 499: Here and elsewhere in Results section I’m surprised to see some methodology details related to concentration calculations. Methods for calculating concentrations should be in an earlier section. If NIST610 was analyzed it would be a simple matter using the Trace Elements DRS in Iolite4.
Line 523: Is 1634.7 ± 1.7 Ma - an error of 0.1% - realistic? Again, how well as the secondary standard measured?
Line 645: “…the mobility of radiogenic 87Sr* is significantly greater than that of Rb and non-radiogenic Sr…” The argument is that 87Sr* is preferentially located in interlayer site which is where alteration processes might start. Preferential removal of 87Sr* would, therefore, lower the 87Sr/86sr to values unsupported on earth. But the least-altered muscovite in basement gneiss also gives initial of 0.6709 ± 0.0115. This doesn’t make sense. The authors claim to have ruled out instrumental effects but is there some cryptic matrix-mismatch adversely affecting corrected 87Sr/86Sr? Some statement about the anomalously low initial in least-altered muscovite would be warranted (unless I missed it somewhere in the text)
Line 697: I see here that the authors explain that age and initial intercept for La Posta were within accepted range and that anomalously low 87Sr/86Sr are not analytical artifacts. It would be nice to know this before launching into the results section where these anomalously low 87Sr/86Sr raise flags about this possibility.
Line 723: typo in isochrone
Line 728: back to x.xx Ga rather than xxxx Ma ages…