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

Abstract

International audienceThe 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 degrees C) at similar to 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 Ar-40-Ar-39 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 x 10(-3) to 10.4 x 10(-3), and molar I/Cl ratios of 1.8 x 10(-6) to 8.2 x 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 Ar-40/Ar-36 of between the modern atmospheric value of similar to 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 nonradiogenic Kr-84/Ar-36 and Xe-129/Ar-36 ratios are slightly enriched in Xe-129 relative to air and the fluid inclusions are estimated to contain 0.5-17.3 x 10(-10) mol g(-1) Ar-36 which is up to twenty times the Ar-36 concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic Ar-40 within K-rich basement rocks, that would have been an important source of excess Ar-40, was limited by temperatures of less than 200 degrees 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 tem...

Basin-related uranium mineral systems in Australia: A review of critical features

“…The presence of halite crystal casts, indicative of evaporites, have been reported in the McKay Sandstone (upper part of the Kombolgie Subgroup) and in the Cottee Formation, overlying rocks of the Kombolgie Subgroup (Kyser and Cuney, 2009;Ferenczi and Sweet, 2004). The analysis of halogen and noble gases and composition of the uraniumbearing fluid inclusions in uranium deposits in the Athabasca Basins show that fluid composition was strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks (Richard et al, 2011(Richard et al, , 2014. It is more than likely that high-salinity fluids documented in the Kombolgie Subgroup sandstones were also at least partially generated by similar processes.…”

Insights into B-Mg-Metasomatism at the Ranger U Deposit (NT, Australia) and Comparison with Canadian Unconformity-Related U Deposits

“…The latter indicate that the unconformity-related U deposits from the Athabasca Basin and the ARUF were formed by highly-saline basinal brines derived from highly-evaporated seawater (up to epsomite saturation) [9,22,23]. The dissolution of evaporites in the Athabasca Basin is probably very limited as indicated by halogens ratios [22]. In addition, low δD values were also measured in fluid inclusions representative of the U-forming brines in the Athabasca Basin and are consistent with seawater evaporation [71].…”

“…Such model is however not supported by the halogen compositions (Cl, Br) of fluid inclusions in Athabasca and ARUF deposits. The latter indicate that the unconformity-related U deposits from the Athabasca Basin and the ARUF were formed by highly-saline basinal brines derived from highly-evaporated seawater (up to epsomite saturation) [9,22,23]. The dissolution of evaporites in the Athabasca Basin is probably very limited as indicated by halogens ratios [22].…”

“…Based on fluid inclusion and stable isotope studies, the mineralising fluids are widely accepted to be 100-200 • C basinal brines, which underwent significant modification by interaction with basement lithologies [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Halogen geochemistry of fluid inclusions indicates that the basinal brines were produced by evaporation of seawater [9,[21][22][23][24]. In addition to basinal brines, a low-salinity fluid of possible meteoric origin was also involved at the time of U deposition in the Australian deposits [9,10].…”

Lithium isotopes in fluid inclusions as tracers of crustal fluids: An exploratory study

“…(b) A C C E P T E D M A N U S C R I P T 28 Samples from the Panasqueira W-Sn deposit (Portugal) are paragenetically equivalent or the same samples as from the "main sulfide stage / pyrrotite alteration stage" (MSS/PAS) (Polya et al, 2000). (Richard et al, 2014); Br/Cl and I/Cl values of the four samples from Southwest England (HW1068F, SW18, WCO, and FI37) and the two samples from Panasqueira Sn-W deposit have been obtained from ion chromatography in this study. Missing I/Cl values indicate that iodine was below limit of detection (except for sample CA 00-1 for which no analysis was done).…”