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 δ18O 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 δ18O 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 1M type crystallized from chemically different fluids, at slightly lower temperatures and away from U concentrations than the equivalent trans-vacant 1M type detected next to the U ores, both precipitating contemporaneously within analytical uncertainty. In addition, comparison of the δD 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.