Search

The Phoenix uranium deposit in the southeastern Athabasca Basin (Canada) is a typical unconformity contact-hosted uranium deposit, characterized by an association with reactivated basement faults, graphite-rich rocks in the underlying basement, and a pervasive clay alteration halo surrounding the mineralized zones. Petrographic results suggest that the mineralizing hydrothermal system was characterized by alternating desilicification and silicification events. Uraninite and clay-size minerals (tourmaline, kaolinite, illite and minor chlorite) mainly precipitated in the desilicification periods whereas hydrothermal quartz (mainly drusy quartz) formed during the silicification periods. Primary fluid inclusions in the hydrothermal quartz are inferred to represent the ore-forming fluids even though quartz did not co-precipitate with uraninite. The coexistence of multiple types of fluid inclusions (liquid-dominated biphase, vapor-dominated biphase, vapor-only and halite-bearing triphase) within individual fluid inclusion assemblages is interpreted to indicate fluid boiling and heterogeneous trapping. Bulk fluid inclusion volatile analysis by mass spectrometry indicates H2O as the dominant species, with less than 1 mol% non-aqueous volatiles that show a compositional trend typical of fluid boiling. Microthermometric and cryogenic Raman spectroscopic analyses indicate that the mineralizing fluids are of H2O-NaCl-CaCl2 ± MgCl2 composition, with salinities ranging mainly from 23.4 to 31.1 wt%. The liquid-dominated biphase inclusions (excluding those interpreted to have resulted from heterogeneous trapping) have homogenization temperatures from 90 to 157 °C. These data are generally consistent with the classical diagenetic-hydrothermal model in which basinal brines that extracted uranium from the basin or the basement were channeled along reactivated basement faults, and precipitated uraninite near the unconformity through reaction with reducing agents. However, the relatively low fluid inclusion homogenization temperatures documented in this paper, together with fluid boiling inferred from fluid inclusion assemblages, suggest that the deposit may have formed in a shallower environment (∼2.5 km) than assumed in the conventional diagenetic-hydrothermal model (>5 km). Abrupt fluid pressure drops during episodic faulting may have resulted in fluid boiling (flash vaporization) changing the fluid pH and promoting uraninite precipitation, with the alternating liquid-vapor conjugates of the boiling system resulting in alternating precipitation of quartz and uraninite respectively.

The Beaverlodge district in northern Saskatchewan is known for “vein-type” uranium mineralization. Most of the uranium deposits are spatially related to major structures, and hosted by ca. 3.2–1.9 Ga granitic rocks (and albitite derived from them) and by ca. 2.33 Ga Murmac Bay Group amphibolite, all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82 Ga Martin Group, and by the flat-lying ca. 1.75–1.5 Ga Athabasca Group. The uranium mineralization is mainly hosted in fault rocks (breccias) and carbonate ± quartz ± albite veins, referred to as breccia-style and vein-style mineralization, respectively, with the latter being the focus of this study. Most of the mineralized veins occur in the basement rocks, although some crosscut the Martin Group. This study examines the field, petrographic, fluid inclusion and C-O isotope characteristics of mineralized and non-mineralized veins from 19 deposits/occurrences as well as from the Martin Group, with an aim to better understand the mineralizing environment and processes. The coexistence of liquid-dominated (L + V), vapour-dominated (V + L) and vapour-only (V) fluid inclusions within individual fluid inclusion assemblages (FIAs) in the veins suggests fluid immiscibility and heterogeneous trapping. The L + V inclusions with the lowest homogenization temperatures (Th) within individual FIAs are interpreted to represent homogeneous trapping of the liquid phase, which yield Th values from 78° to 330 °C (mainly 100° to 250 °C), and salinities from 0.2 to 30.8 wt.% NaCl equivalent. Mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by H2O (average 97.2 mol%), with minor amounts of CO2, CH4, H2, O2, N2, Ar and He. Fluid pressures were estimated to be < 200 bars based on the inference of fluid immiscibility, fluid temperatures of 100° to 250 °C, and low concentrations of non-aqueous volatiles (< 3 mol%). The δ18OVPDB and δ13CVPDB of carbonate minerals associated with mineralization range from − 20.5 to − 8.9‰ and − 10.1 to − 0.9‰, respectively. The δ18OVSMOW values of the parent fluids calculated using the Th values range from − 9.6 to + 17.0‰, with the majority from 0 to + 5.0‰. O isotopes of paired equilibrium quartz and calcite, analyzed by secondary ion mass spectrometry (SIMS), yield temperatures from 161° to 248 °C, which are consistent with the fluid inclusion data. The new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic-hydrothermal model as proposed in some previous studies (for breccia-style and vein-style mineralization), but rather support a model in which the vein-type uranium mineralization took place at relatively low temperature (100° to 250 °C) and shallow (< 2 km) conditions, with fluid pressure fluctuating between hydrostatic and sub-hydrostatic regimes, possibly related to episodic faulting. The mineralizing fluids were mainly sourced from the Martin Lake Basin, and uraninite was precipitated as a result of mixing between this basin-derived fluid and fluids carrying reducing agents (Fe2 +, CH4) derived from the basement, although fluid-rock reactions and fluid immiscibility may have also played a role.