Milidragovic and five co-authors reported in 2015 that the Otish Basin of central Québec hosts over thirty uranium prospects, including the Camie River prospect that is located near the unconformable contact between the graphite-bearing, metamorphic basement of Archean age and the overlying basinal sedimentary rocks of the Otish Supergroup.
Geological map of the Otish Basin (modified from Lesbros-Piat-Desvial, 2014),
showing select uranium showings (stars). Inset: location of Otish Basin in
relation to the Superior Province.
The maximum age of the basin, constrained by the age of the unconformably-underlying Mistassini dyke swarm, is 2515 ± 3 Ma. Following the deposition of the Otish Supergroup, the basin was intruded by the Otish Gabbros, a suite of olivine-tholeiitic dykes and sills of near-liquid compositions (Group 1) and cumulate rocks (Group 2). Uranium-lead zircon ages from the Otish Gabbros indicate the minimum age of the Otish Basin is ca. 2.17 Ga. The Otish Basin was also intruded by the Matoush dyke, interpreted to be younger than the Otish Gabbros. The least altered samples of the Matoush dyke have mineralogy and trace element systematics, such as highly fractionated REE patterns, similar to lamprophyric rocks and are distinct from the tholeiitic to weakly trace element-enriched Otish Gabbros. This, therefore, indicates that the Otish Basin was affected by at least two igneous events.
The uranium mineralization formed after the peak diagenetic alteration of the Otish Supergroup. During peak diagenetic conditions, early albitic sandstone cement was largely replaced by K-feldspar. The feldspathic cement was subsequently partially replaced by a green muscovite alteration. The increase in Na2O concentrations of the feldpsathiccemented sandstones towards the base of the sedimentary sequence is noteworthy. Uraninite and brannerite are the principal uranium minerals at the Camie River prospect.
Molybdenite grains, intergrown with uraninite, yield a Re-Os model age of 1724 ± 4. 9 Ma that is indistinguishable from the previously published uraninite ages. The age of mineralization is significantly younger than the age of the Otish Gabbros, indicating that the uranium mineralization postdates sedimentation by ≥450 m.y. The overlapping Sm-Nd isotopic ages from Otish Gabbros suggest the main uranium mineralization was locally accompanied by resetting of the Sm-Nd system and LREE mobility in the gabbros.
The age of mineralization in the Otish Basin is similar to the age of polymetallic mineralization of the Huronian Supergroup, suggesting the ca. 1.7 Ga hydrothermal activity may have been a regional phenomenon affecting the southern Superior Province.
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