The clay-size fraction of the alteration halo associated with the Cigar Lake uranium deposit consists of illite with subordinate kaolinite, sudoite (Al-chlorite), Fe-chlorite, hematite, and traces of an illite–chlorite mixed-layer mineral. The Kübler Index of illite crystallinity ranges from 2.0 to 6.7; the most-altered samples average 4.0. These data indicate that high-grade diagenetic to anchizonal conditions have been reached within the alteration halo. Intensity ratios of the 002 and 001 illite reflections (I(002)/I(001)) are generally greater than 0.4, the phengite–muscovite boundary, reflecting aluminum-rich illites; illite composition approaches hydromuscovite. Whereas 1M and 2M polytypes prevail regionally, the presence of 3T and 2M polytypes in the alteration halo suggests that, locally, higher temperatures have been attained than the regional estimates of 180–220 °C.K–Ar dates for illite-rich samples, from 1255 to 1148 Ma, could reflect variable resetting of the K–Ar system during hydrothermal fluid circulation subsequent to and (or) during main-stage uranium-ore formation. The sudoite–illite assemblage from an altered basement sample yields a date of 815 Ma, unlike the clay-rich ore sample that contains no radiogenic argon. The younger date of the basement sample and the lack of radiogenic 40Ar in the ore sample may result from alteration.Average δD and δ18O values for illite-rich samples are −82 and +10, respectively. Samples that contain sudoite are enriched in D, but the ore sample containing Fe-chlorite is depleted in D and 18O relative to the illite-rich samples. The variation observed in δD values may reflect the exchange behaviour of different clay minerals or low-temperature interaction with meteoric waters.