Noble gases (Ar, Kr, Xe) and halogens (Cl, Br, I) in fluid inclusions from the Athabasca Basin (Canada): Implications for unconformity-related U deposits

• Brines involved in Proterozoic U deposits (Athabasca, Canada) are targeted.
• Halogens (Cl, Br, I) indicate a subaerial evaporation of seawater origin for brines.
• Brines acquired excess noble gases (Ar, Kr, Xe) by interaction with clastic sediments.
• Interaction with basement-derived fluid or organic matter is unlikely.

The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada) involved basin-scale circulation of U-bearing brines during high-grade diagenesis (150–200 °C) at ∼1.6–1.5 Ga. The UO2 ores occur both sides of the unconformity and are associated with extensive brecciation and illite–sudoite–dravite alteration. Quartz and dolomite cementing veins and breccias are associated with alteration and mineralisation and contain a fairly uniform population of fluid inclusions characterised by variable Na:Ca and salinities of 25–35 wt.% salts and high U concentrations of up to 600 ppm U. In order to further constrain the origin of these U-rich brines, we analysed the naturally occurring isotopes of Ar, Kr and Xe, together with halogens (Cl, Br and I), K, Ca and U in irradiated quartz and dolomite samples containing representative fluid inclusions. This was achieved by the noble gas method for halogen measurement (extended 40Ar–39Ar methodology) using a combination of noble gas extraction techniques.The fluid inclusions opened by crushing quartz and dolomite samples in vacuum have similar molar Br/Cl ratios of 5.8 × 10−3 to 10.4 × 10−3, and molar I/Cl ratios of 1.8 × 10−6 to 8.2 × 10−6. These compositions lie over the top half of the modern-day seawater evaporation trajectory, consistent with the fluids deriving the bulk of their salinity by subaerial evaporation of seawater, beyond the point of halite saturation. The I/Cl ratios are much lower than is typical of fluids that have interacted with I-rich organic matter present in many sedimentary basins or fluid inclusions found in Mississippi Valley type (MVT) Pb–Zn ore deposits. This is significant because provided the U-rich fluid inclusions are representative of the ore-stage fluids, the low I/Cl ratios of the fluid inclusions do not favour fluid interaction with organic matter (or hydrocarbons), as a major process for localising U mineralisation.The majority of samples contain fluid inclusions with age-corrected 40Ar/36Ar of between the modern atmospheric value of ∼300 and 450. These values are considered representative of the fluid's initial composition and are typical of upper crustal sedimentary formation waters. The fluid inclusions non-radiogenic 84Kr/36Ar and 129Xe/36Ar ratios are slightly enriched in 129Xe relative to air and the fluid inclusions are estimated to contain 0.5–17.3 × 10−10 mol g−136Ar which is up to twenty times the 36Ar concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic 40Ar within K-rich basement rocks, that would have been an important source of excess 40Ar, was limited by temperatures of less than 200 °C.Taken together the halogen and noble gas composition of the U-bearing fluid inclusions are strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks, showing that low temperature evaporitic brines dominated the mineralising system. Mineralisation is unlikely to have been triggered by fluid interaction with organic matter, or mixing with voluminous basement-derived fluids; however, the data do not completely preclude a role for volumetrically minor fluid or gas phases introduced by deep-seated basement faults preferentially located at the sites of mineralisation.