The uranium mineralization potential of the Paleoproterozoic Sioux Basin and its relationship to other basins in the southern Lake Superior region

Abstract

The Paleoproterozoic Sioux Basin and the Baraboo, Barron, Flambeau, McCaslin, and Waterloo quartzites in northcentral USA are all supermature quartz arenites that are among the world's oldest redbeds. These quartzites were deposited on a stable craton at ca. 1750 Ma, following exhumation and uplift of Penokean crust and major orogenic activity related to the amalgamation of Archean microcontinents at ca. 2.0–1.8 Ga to form Laurentia. 40Ar/ 39Ar ages of 1615–1543 Ma from diagenetic muscovite in the Sioux Basin provide the first geochronological evidence for Mazatzal-related deformation and low-grade metamorphism of the quartzites during subsequent convergence on the southern margin of Laurentia. One muscovite sample from the Sioux Basin has a 40Ar/ 39Ar age of 1465 Ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40Ar/ 39Ar ages for muscovite from the Baraboo and Waterloo quartzites. Thus, the Sioux Basin and other quartzites in the region have experienced a similar tectonic and fluid history. The Sioux Basin has similar sedimentologic characteristics as other Paleoproterozoic basins in North America, including the uranium-rich Athabasca Basin, Canada, that is host to some of the world's largest and highest-grade unconformity-type uranium deposits. We compared key characteristics, such as the stratigraphy, character of diagenetic fluids, paleoaquifer characteristics, and composition of leachable Pb in the sandstones from the Sioux Basin with the Athabasca Basin in order to evaluate the fluid history of the basin and the potential of the Sioux Basin to host unconformity-type uranium mineralization. Based on the hydrogen and oxygen isotopic composition of muscovite (M1) that formed during diagenesis at 150–200 °C, the pore fluids that were present during peak diagenesis in the Sioux Basin had δ 18O and δD values of −0.5 to +5.2‰ and −32 to −46‰, respectively, similar to those of the fluids associated with uranium mineralization and alteration in the Athabasca Basin. However, fluid flow in the Sioux Basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers, in contrast to the extensive paleoaquifers in the Athabasca Basin that promoted large-scale diagenetic reactions and alteration that led to the formation of unconformity-type uranium mineralization. In addition, the non-radiogenic isotopic composition of leachable Pb in the Sioux Basin sandstones indicates that the majority of the Pb is from a normal crustal source, inconsistent with mobilization of radiogenic Pb from a uranium-rich source, such as a uranium deposit. Although the Sioux Basin shares similar general characteristics to the Athabasca Basin, the results of this study suggest that it has a lower potential than the Athabasca Basin to host significant high-grade unconformity-type deposits.