The Thelon Formation, uppermost unit of the Dubawnt Group, overlies a regionally extensive paleoweathered zone developed on a wide range of lithochronological units including formations in the lower Dubawnt Group. Authigenic uraniferous phosphate minerals, fluorapatite and goyazite, cementing Thelon conglomerate–sandstone and filling fractures in the underlying paleoweathered zone, were dated in an attempt to better constrain the age of Thelon sedimentation and diagenesis. The oldest age, 1720 ± 6 Ma, derived from phosphate-cemented sediments, is interpreted as a minimum age for diagenesis and therefore brackets initial Thelon sedimentation between emplacement of fluorite-bearing granites at 1753 Ma and authigenic phosphate cementation at 1720 Ma. Additional ages of 1685 ± 4 and 1647 Ma are interpreted as remobilization or subsequent cementation events. K–Ar ages on illite, 1386 ± 37 and 1266 ± 31 Ma, from the paleoweathered zone and basal conglomerate, respectively, are significantly younger than ages derived from coexisting phosphate. These K–Ar ages record hydrothermal events that may be related to processes associated with unconformity-type uranium mineralization at approximately 1400–1300 Ma.Ages from the Thelon Basin permit geochronologic correlations with the Athabasca and Hornby basins, long correlated on the basis of similarities in sedimentation, stratigraphy, and tectonic setting.

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Summary Extensive potassic alkaline magmatism accompanied early stages in the formation of intracratonic basins in central Keewatin. Subaerial trachyandesite and trachyte flows, minette, syenite and bostonite dykes and small pyroxenite-syenite stocks comprise the c. 1.8 Ga igneous suite. High Mg, Cr and Ni contents in mafic flows and mica-rich lamprophyres indicate crystallization from mantle-derived magmas. Enhanced LIL element content and LREE enriched noninflected REE patterns are consistent with an enriched upper mantle source containing high-Ti micas. Enrichment in Sr, K, Rb, Ba and Th is interpreted as due to mobilization by water-rich fluids. Polymodal zoned clinopyroxene and mica macrocrysts in lavas and dykes record combined effects of crystal fractionation, disaggregation of wall rock xenoliths and magma mixing during rapid magma ascent through extending continental lithosphere. High Fe-Ti-Ba xenocryst micas may have originated metasomatically in the upper mantle where they were incorporated as partly melted crystals near the site of subsequent magma generation. Inhomogeneous lithospheric reworking and cracking during ç 1.9–1.85 Ga collisions may be responsible for post-tectonic extension and the voluminous alkaline magmatism in central Keewatin. A wide spectrum of uranium mineralization and minor gold and base metal occurrences are found in rocks of the alkaline suite. Syngenetic mineralization is limited to disseminated REE+U+Th in bostonite dykes. Minor U±Cu±Ag±Pb mineralization results from thermal and hydrothermal effects of dyke and pluton intrusion. Epigenetic uranium mineralization is associated with localized propylitic alteration of the alkaline rocks and with crosscutting quartz veins, fracture systems and fault zones. Important metal associations are U+Cu+Pb+Ag±Mo±Zn, U+Cu+Se+Ag+Au and Pb+Cu±Zn±Cd±Ag±Bi.

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