Characterization of Fluids Associated with Uranium Mineralization in the Beaverlodge Uranium District, Northern Saskatchewan: Field, Petrographic, Fluid Inclusion and C-O Isotope Studies

English Abstract:

The Beaverlodge uranium district north of Lake Athabasca in northern Saskatchewan is known for vein-type uranium mineralization in the pre-Athabasca basin basement. Most of the uranium deposits are distributed along major structures, and hosted by granitic rocks of various ages (and albitite derived from them) and by the ca. 2.33 Murmac Bay group amphibolite, both of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82 Martin group. The uranium mineralization is mainly developed in breccias and carbonate±quartz veins. Three types of fluid inclusions were found in the veins, liquid-dominated biphase, vapour-dominated biphase, and vapour-only monophase, suggesting boiling or phase separation. The liquid-dominated fluid inclusions have homogenization temperatures from 57^o to 329 ^oC (mainly 150^o – 200 ^oC) and salinities from 0.2 to 31.4 wt% NaCl equivalent (clustered in two ranges, 0.2 – 7.0 and 20.0 – 31.4). Mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by H₂O, with minor amounts of CO₂, CH₄ and H₂; the data plot in an area overlapping those of basinal brines and meteoric water. The δ¹⁸O and δ¹³C values of carbonates in the veins range from -22.5 to -6.6‰ (PDB) and from -10.1 to +1.1‰ (PDB), respectively. δ¹³C values from the Martin group (-2.7 to +1.1‰) are among the highest, whereas those from some deposits are relatively low (-10.1 to -8.2‰). The calculated δ¹⁸O values of the parent fluids mainly range from 0 to +5.0‰ (SMOW), lower than magmatic and metamorphic fluids. These data suggest that the uranium mineralization was formed in a relatively shallow environment, where brines from the Martin Lake basin and meteoric water may have been channelled along high permeability zones produced by structural deformation and albitization, and mixed with fluids from the basement containing Fe²⁺ and methane. Fluid mixing, fluid-rock interaction, and boiling may have caused precipitation of uraninite.