Paleoregolith and Unconformity-type Uranium Mineralization, Beaverlodge Lake, Great Bear Magmatic Zone, Northwest Territories

Abstract

During the Paleoproterozoic Era (ca. 2.5 Ga to 1.6 Ga), Earth underwent dramatic changes to its tectonic and atmospheric parameters. These changes included: the formation and breakup of the supercontinent Nuna (Columbia) and the gradual rise in atmospheric oxygen levels. The gradual rise in atmospheric oxygen, referred to as the Great Oxidation Event (GOE), altered the behaviour of silicate mineral weathering, and permitted the formation of new types of economic uranium deposits. Beaverlodge Lake, Northwest Territories (NT), allows for the study of a weathering profile and uranium mineralization post GOE.

At Beaverlodge Lake, NT, a regolith is preserved in a rhyodacitic porphyry of the ca. 1.93 Ga Hottah plutonic complex, which is unconformably overlain by the ca. 1.9 Ga quartz arenite of the Conjuror Bay Formation. Coincident with the unconformity is a past-producing uranium deposit (called the Tatie U deposit), which was mined out in the 1930s. Other uranium showings have been discovered at Beaverlodge Lake including the Bee showing. The initial purpose of this project was to examine the regolith through field, petrography, electron microprobe analysis (EMPA), whole-rock geochemistry, and mass balance calculations. The weathering profile shows an increase in Al2O3, Fe2O3T, K2O, P2O5, Ba, and Rb, a loss in SiO2, Na2O, MgO, and Sr, and constant and low abundance of CaO. Titanium remains constant in the weathering profile. Rare earth element (REE) analysis reveals remobilization of light REE (LREE) on a micrometer scale, but no cerium anomaly is preserved in the weathering profile. The weathering profile displays characteristics similar to other post GOE paleoweathering profiles developed on felsic parental material.

The timing of uranium mineralization at Tatie and Bee was constrained by in-situ U-Pb uraninite dating by Secondary Ion Mass Spectrometer (SIMS), which yielded two variably discordant ages of 1370.2 ± 7.9 Ma and 407 ± 21 Ma. In addition, REE contents of uraninite were determined by in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Results revealed two types of uraninite mineralization are preserved at Beaverlodge Lake and they consist of synmetamorphic at Bee and basement-hosted unconformity-type at Tatie similar to those in the Athabasca Basin. The ca. 1370 Ma uraninite (Tatie) is characterized by an asymmetric bell-shaped REE pattern centered on Tb to Er where LREEs are depleted compared to heavy REEs (HREE). The ca. 407 Ma uraninite at Bee has low La concentrations and a flat to slightly negative REE pattern. The Mesoproterozoic age is similar to a Pb loss age of ca. 1400 Ma found in the Athabasca Basin. The younger Devonian age may be related to meteoric fluids cycling and uranium remobilization during the Phanerozoic.