K–Ar dating and δ18O–δD tracing of illitization within and outside the Shea Creek uranium prospect, Athabasca Basin, Canada

Abstract

Abstract Isotope analyses (K–Ar, δ 18 O and δ D) were performed on illite from both the sandstone cover and the underlying basement, close to and distant from Shea Creek, an unconformity-type U deposit (Athabasca Basin, Canada); the illite had previously been characterized crystallographically. In the barren areas away from deposit, illite is mainly of the cis-vacant 1 M polytype occurring as relatively coarse-grained lath-shaped particles, while it occurs as fine-grained particles of the trans-vacant 1 M type next to and in the U mineralized strata. The tectonic-induced hydrothermal system that favored illite crystallization was multi-episodic 1453 ± 2, 1330 ± 20 and probably about 1235 Ma ago. These illite-forming episodes appear to have occurred contemporaneously to those favoring the concentration of the associated U oxides, which were dated independently by the U–Pb method in the Shea Creek deposits and elsewhere in the Athabasca Basin. No relationship was found between the illite polytypes and their crystallization ages, meaning that precipitation of each, either as the cis-vacant or the trans-vacant type, did not relate to a specific event, but to variable physical and chemical crystallization conditions during the same event. The change in the contemporaneous illite polytypes appears to relate to an increase in the δ 18 O with distance to the U deposit. Such a change could result from a progressively lower formation temperature with increasing distance to the U deposit, probably combined to a changing δ 18 O of the interstitial fluids due to variable water–rock interactions in the rocks. Variable water–rock ratios could have resulted from variable tectonic adjustments of the basement. The authors are inclined to believe that the cis-vacant 1 M type crystallized from chemically different fluids, at slightly lower temperatures and away from U concentrations than the equivalent trans-vacant 1 M type detected next to the U ores, both precipitating contemporaneously within analytical uncertainty. In addition, comparison of the δ D of the hydroxyls from cis-vacant and trans-vacant types suggests that the illite mineral structure was not affected by radiation related to radioactive decay within the deposit or by further natural alteration, as advocated for other occurrences.