The Almadin mercury deposits represent one of the largest geochemical anomalies on Earth, but the mode and timing of their formation remains a matter of controversy. There are two main possibilities: hydrothermal solutions associated with alkali basalt volcanism in the Silurian and Devonian; or regional hydrothermal activity during later Hercynian metamorphism. Although these models can be distinguished by determining the age of ore deposition, no suitable isotopic method had been applied to the problem prior to this study. We report here on vacuum encapsulated and more traditional laser "Ar/ "Ar dating performed on illite concentrates and Cr-rich micas that are intimately associated with mercury mineraiization in both Devonian and Silurian host rocks. Illite is associated with a later stage of Hg mineralization (cinnabar + kaolinite + pyrophyllite + chlorite) which locally replaces an earlier episode associated with carbonates and Cr-mica. The dating results from illite separates suggest that the later stages of Hg mineralization or remobilization occurred about 360 Ma, which is about 20 Ma younger than any mercury-hosting rocks in the district and coincides with the onset of regional deformation during the Hercynian orogeny. Cr-mica ages are variable and range from 365 Ma to 427 Ma. This age range spans the period between the deposition of the Criadero quartzite (the oldest sedimentary unit hosting mercury), and the later episode of mineralization. This suggests that mineralization started at least by the mid to lower Silurian and the younger argon ages for Cr-mica could represent partial to nearly total argon loss, caused by the Hercynian metamorphic event. It appears that both postulated styles of mercury mineralization occurred, separated in time by up to 80 million years.

The McClean group of uranium deposits consists of elongate pods of high-grade uranium mineralization (width = ~ 15–40 m) tightly confined to within ±40 m of the basal unconformity. Uraninite–coffinite–sulphide/arsenide–chlorite–siderite mineralization at McClean is surrounded by a muscovite/illite ± haematite hydrothermal alteration halo,which can contain coffinite–pararammelsbergite (NiAs2) – muscovite/illite nodules. Ten laser probe 40Ar/39Ar dates, two of which are step-heat runs showing good plateaus, and 10 conventional K – Ar dates for this material show a distribution with asharp beginning at ~ 1320 Ma, a marked peak in the 1250–1200 Ma class interval, and a tail to dates as young as 1002 ± 33 (1σ) Ma. These determinations are in complete agreement with direct (U–Pb and Sm–Nd) dates on uraninite at the Midwest (e.g., 1328 ± 9 and 1110 ± 28 Ma), Key Lake (e.g., 1350 ± 4 and 1281 ± 6 Ma), and Collins Bay B deposits(e.g., 1281 ± 80 Ma). Since estimated depositional ages for the Athabasca sedimentary sequence are in the 1470 ± 15 to1428 ± 15 Ma range, uranium mineralization and associated hydrothermal alteration started ~ 100–150 Ma after Athabasca sedimentation, a result consistent with fluid-inclusion data, which indicate that mineralization took place at ~ 160–220 °C beneath ~ 3000 m of cover at a relatively advanced stage in the evolution of the basin. It is suggested that the similar initiation dates for uranium mineralization might reflect a widespread faulting event that affected the eastern part of the basin. A muscovite/illite closure temperature calculated from a measured argon diffusion activation energy of 36 ± 4 kcal/mol(1 kcal = 4.1868 kJ) indicates that the base of the Athabasca Basin in the McClean area has not been disturbed by temperatures greater than ~ 140 °C for 1.1–1.0 Ga. It is suggested that mineralization ceased when fracture permeability had been sealed by crystallization of secondary minerals. The duration of mineralization may have been ~ 150 Ma, a relatively long time interval not unreasonable for the base of a sedimentary basin.Secondary illites interstitial to quartz grains from the HLM1 stratigraphic borehole give 40Ar/39Ar ages of 1459 ± 4, 1341 ± 4, and 1113 ± 11 Ma, indicating that formation of diagenetic sheet silicates predated uranium mineralization. Recrystallization or formation of sheet silicates in relict sedimentary layers and in subunconformity altered basement referred to as "regolith" started at approximately the same time, since dates of 1484 ± 55 Ma (sedimentary layer), 1482 ± 49 Ma (regolith), and 1453 ± 49 Ma (regolith) have been obtained. Resetting of interstitial, sedimentary layer, and regolith sheet-silicate dates continued to ages of, for example, 1113 ± 11 Ma (interstitial) and 1038 ± 55 Ma (sedimentary layer), which exactly coincide with the youngest ages obtained for the alteration halo associated with mineralization.The youngest date obtained is a 40Ar/39Ar plateau age of 673 ± 3 Ma. The sample (2045-517) was obtained from within 2 mm of a concentrated pitchblende nodule and may have been disturbed in some way by its proximity to uranium.

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