Characterization of Fluids Associated with Vein-type Uranium Mineralization in the Beaverlodge Uranium District, Northern Saskatchewan: Field, Petrographic, Fluid Inclusion and C-O Isotope Studies
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The Beaverlodge uranium district north of Lake Athabasca in northern Saskatchewan is known for vein-type uranium mineralization hosted in Archean to Proterozoic granitic and metamorphic rocks. Most of the Beaverlodge deposits are spatially related to major structures, and hosted by granitic rocks of various ages (and albitite derived from them) and by ca. 2.33 Ga Murmac Bay group amphibolite, both of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82 Ga Martin group, and by the weakly deformed to non-deformed ca. 1.75-1.5 Ga Athabasca group. The uranium mineralization in the Beaverlodge uranium district is mainly developed in fault rocks and carbonate ± quartz veins. The compositions of the vein minerals vary from one deposit to another, including variable amounts of quartz, Fe-poor calcite, Fe-rich calcite, Fe-poor dolomite and Fe-rich dolomite. Most of the mineralized veins are developed in the basement rocks, and some crosscut the Martin group. Three types of fluid inclusions were found in the veins, including liquid-dominated biphase, vapour-dominated biphase and vapour-only monophase inclusions. The coexistence of these three types of fluid inclusions within individual fluid inclusion assemblages suggests fluid boiling or phase separation. Microthermometric measurements of liquid-dominated fluid inclusions show homogenization temperatures (Th) from 78o to 330oC (mainly 100o to 250oC), and salinities from 0.2 to 30.8 wt% NaCl equivalent (clustered in two ranges, 0.2-5.9 wt% NaCl equiv and 18.2-30.8 wt% NaCl equiv). The average homogenization temperature of liquid-dominated inclusions in quartz from the mineralized veins of one deposit (Hab, 231.5oC) is very similar to the calculated temperature (238.3oC) obtained from chlorite geothermometry. Mass spectrometric analysis of bulk fluid inclusions shows the volatiles are dominated by H2O, with minor amounts of CO2, CH4, H2, O2, N2, Ar and He. On plots of Ar/He versus N2/Ar, the data overlap the fields of basinal brines and meteoric water. Fluid pressures were estimated by using fluid inclusion isochores in the H2O-NaCl system. The calculated fluid pressures mainly range from 5 to 123 bars, corresponding to a hydrostatic load of 46 to 1256 m for the vein formation and mineralization. The oxygen and carbon isotopes of carbonate minerals associated with uranium mineralization from various occurrences, including those hosted in leucogranite or albitite, amphibolite, and the Martin group, show δ18O ranging from -20.5 to -8.9‰ (VPDB) and δ13C values from -10.1 to -0.9‰ (VPDB). By using the different ranges of homogenization temperatures of fluid inclusions in carbonates, the calculated δ18O values of the parent fluids in equilibrium with the carbonates range from -9.6 to +17.0‰, with the majority from 0 to +5.0‰ (VSMOW), lower than magmatic and metamorphic fluids. It is proposed that the main uranium mineralization in the Beaverlodge uranium district is syn- to post-Martin group, and resulted from the circulation of oxidizing basinal fluids from the Martin Lake basin into the basement at shallow depths. Such fluids were likely channelled along high permeability zones produced by structural deformation, where mixing with fluids from the basement containing Fe2+ and methane, together with fluid phase separation or boiling, caused precipitation of uraninite.