CO2, CH4, C2H6, N2 - ScienceDirect.com

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ScienceDirect Procedia Earth and Planetary Science 17 (2017) 273 – 276

15th Water-Rock Interaction International Symposium, WRI-15

Radiolytic (H2, O2) and other trace gases (CO2, CH4, C2H6, N2) in fluid inclusions from unconformity-related U deposits Antonin Richarda,1 a

Université de Lorraine, CNRS, CREGU, GeoRessources lab., Boulevard des Aiguillettes B.P. 70239, F-54506, Vandoeuvre-lès-Nancy, France

Abstract Fluid inclusions from quartz and dolomite veins from five unconformity-related uranium deposits (Athabasca basin, Canada) have been analyzed by Raman spectrometry in order to identify trace gases in their vapor phase at room temperature. About 80% of fluid inclusions have detectable gases. The most common gases are H2, O2, CO2, CH4, C2H6 and N2. So-called “NaCl-rich” and “CaCl2-rich” brine inclusions have similar gas contents. U-bearing fluid inclusions have similar gas compositions when compared to U-absent fluid inclusions. Radiolysis (i.e. production of H2 and O2 from H2O when fluids have been in contact with previously deposited UO2) and fluid-rock interactions are the most probable origins for H2, O2 and CO2, CH4, C2H6 and N2 respectively. The relative abundance of radiolytic O2 is related to distance to ore and could be used as vectors towards mineralization. Other trace gases may be indicative of ore-forming processes, specifically of fluid-rock interaction and UO2 deposition. 2017The TheAuthors. Authors. Published by Elsevier © 2017 Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Raman; fluid inclusion; uranium deposit; gas; radiolysis; Athabasca basin

1. Introduction Trace gases in fluid inclusions have been widely documented around high-grade unconformity-related U deposits in the Athabasca basin (Canada). H2 and O2 have been interpreted as resulting from water radiolysis in contact with UO2 ore1-3. Differential migration of H2 and O2 has been demonstrated for the McArthur River deposit, as fluid inclusions located at more than 10m from high-grade ore show H2 while O2 is not detected3. Other trace gases (CO2, CH4, C2H6, N2) have a more debatable origin; water-rock interaction in graphite-bearing meta-pelites being the most frequently favored hypothesis4-7. Most authors acknowledge the critical importance of radiolytic and other trace * Corresponding author. Tel.: +33-383-684-737; fax: +33-383-684-701. E-mail address: [email protected]

1878-5220 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.053

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Antonin Richard / Procedia Earth and Planetary Science 17 (2017) 273 – 276

gases as potential prospecting tools and as evidence of fluid-rock and fluid-gas interaction possibly at the origin of UVI reduction in the ore-forming brines and UO2 deposition3,7,8. Here, I provide new gas compositions of the vapor phase of fluid inclusions at room temperature for four U deposits (Shea Creek, Rabbit Lake, Eagle Point, P-Patch), using samples located at more than 10m from high-grade ore, as a complement to previously published data on McArthur River3. In addition, I provide gas composition for fluid inclusions from McArthur River in which U concentrations have been determined by LA-ICPMS9,10. The new data provide for the first time an overview of gas compositions in fluid inclusions from U deposits throughout the Athabasca basin. 2. Geological background, sampling and methods The Na>Ca>Mg>K) and a “CaCl2-rich brine” end-member (Cl>Ca≈Mg>Na>K)9,12-14. Gas species in the vapor phase of fluid inclusions were determined at room temperature with a Labram Raman microspectrometer equipped with a Edge filter, a holographic grating with 1800 grooves per millimetre and a liquid nitrogen cooled CCD detector at GeoRessources (Nancy, France)15. The exciting radiation at 514.5nm provided by an ionized Argon laser (Spectra physics) was focused on the vapor phase of the fluid inclusions using a X80 objective (Olympus). Using this setup, neither limits of detection nor absolute concentrations of trace gases, can be determined. However the relative proportions of the different gas species at room temperature can be estimated qualitatively15 (Fig. 1a,b). 3. Results and discussion Eighty one fluid inclusions were analyzed (Shea Creek: N = 28, Rabbit Lake: N = 17, Eagle Point: N = 16 and PPatch: N = 20) and about 20% show no detectable gas in their vapor phase. H2 is the most frequently detected gas (in ca. 60-80% of gas-bearing inclusions), followed by CH4 (ca. 35-80%), N2 (ca. 20-60%) and CO2 (ca. 5-50%) (Fig. 1d). C2H6 was detected only at Eagle Point (ca. 30%) and O 2 was never detected (Fig. 1c). CO2 is remarkably frequent at Rabbit Lake (ca. 50%) compared to the other deposits (5-10%). There is no significant difference of gas compositions between fluid inclusions classified as “NaCl-rich brines” and “CaCl2-rich brines”. When H2 and C2H6 are taken separately (as H2 is thought to originate from radiolysis and C2H6 is minor), fluid inclusions are dominated by CH4-N2 (CH4 being predominant in most cases). A minority of fluid inclusions have detectable CO2 together with CH4-N2 or CO2 only (Fig. 1e). Fluid inclusions from McArthur River, classified as “NaCl-rich brines” and “CaCl2rich brines”, in which U concentration was detected and quantified by LA-ICPMS commonly have H2 and O2, with rarer CH4 and C2H6, and no CO2 and N2 (Table 1). Fluid inclusions from Shea Creek, Rabbit Lake, Eagle Point and P-Patch differ from those from McArthur River in that they frequently contain N 2 and are devoid of O2 (Fig. 1d)3. Alteration of NH4-bearing feldspar and biotite in the basement and formation of NH 4-bearing micas in the clay alteration haloes are probably the dominant control on the N 2 concentration in the fluid inclusions6. The difference of N2 composition between McArthur River and the other deposits may be attributed to subtle changes in biotite, feldspar and mica compositions that would require further investigation. The absence O 2 in fluid inclusions from the other deposits compared to those of McArthur River is attributable to the distance between samples and high-grade

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ore which is here always greater than 10m for the other deposits3.

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Fig. 1. Gas composition of the vapor phase of fluid inclusions at room temperature for the Eagle Point, Shea Creek, Rabbit Lake and P-Patch deposits. (a) Typical two-phase (liquid + vapor) fluid inclusion in quartz at room temperature. Laser was focused on the vapor phase. (b) Example of Raman spectrum showing distinct peaks for O2, CH4 and H2. (c) Pie chart showing relative proportions of fluid inclusions in which gases were detected and not detected. N = number of fluid inclusions analyzed. (d) Frequency histogram of detected gas species in gas-bearing fluid inclusions for each deposit (i.e. a frequency of X% for a given gas species means that this gas was detected in X% of the gas-bearing fluid inclusions). (e) Ternary diagram showing relative proportions of CH4, CO2 and N2 for all deposits (H2 and C2H6 are considered separately). Table 1. Relative gas composition of some U-bearing fluid inclusions at room temperature from the McArthur River deposit. U concentrations in fluid inclusions and fluid types have been determined by LA-ICPMS and microthermometry respectively9,10. Gas concentrations are given in relative mole % in the vapor phase of fluid inclusions at room temperature. Distance to U (m) ~ 10 ~1 ~ 10 ~ 10 ~ 0.5 ~1

Fluid type NaCl-rich brine NaCl-rich brine NaCl-rich brine NaCl-rich brine CaCl2-rich brine CaCl2-rich brine

U (ppm) 1 8 20 42 34 634

CO2 0 0 0 0 0 0

CH4 15 0 68 0 0 0

N2 0 0 0 0 0 0

H2 85 0 30 67 60 35

O2 0 100 0 5 40 65

C2H6 0 0 2 28 0 0

H2 is present in fluid inclusions from all deposits, even in samples located at more than 10 meters from highgrade ore. This confirms that H2 may migrate over distances of several tens of meters away from the ores 3. There is no strict relation between U concentration and gas composition at McArthur River, in particular H2 and O2 (Table 1). The presence of O2 in those U-bearing fluid inclusions is attributed to the proximity of high-grade ore3. In addition, similar U concentrations have been determined in fluid inclusions from samples taken away from the ore in the other deposits12 and only H2 and no O2 was detected. This would mean that internal radiolysis (i.e. radiolysis within the fluid inclusions due to their high U content) cannot solely explain the H2-O2 composition of those fluid inclusions and that differential migration (possibly by diffusion of H2) and heterogeneous trapping of radiolytic gases in fluid inclusions are required3,16. CO2, CH4, C2H6 and N2 are most probably the result of interaction of oxidizing basinal brines with basement graphite-bearing metapelites. The occasional occurrence of CO2-dominated gas compositions suggests that CO2 was produced independently from CH4 and N2, during episodic pulses of graphite destabilization. This would be compatible with the slightly higher CO2 content of dolomite-hosted fluid inclusions compared to quartz-hosted inclusions5. CO2 in dolomite-hosted fluid inclusions has been analyzed isotopically and δ13C values that range between -30‰ to -4‰ indicate that CO2 originates from brine-graphite interaction in basement metapelites5.

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According to other authors, CO2 and CH4 could both be the products of brine-graphite interaction4,7. The similar composition of the “NaCl-rich” and “CaCl2-rich” brine inclusions is compatible with the fact both brine types underwent extensive fluid-rock interaction in the basement12. Finally, the gas compositions reported here are at variance with those from similar unconformity-related U deposits of the Kombolgie basin (Australia) where “NaCl-rich brine” inclusions have CH4 > N2 > H2 ± O2 while “CaCl2-rich brine inclusions” have N2 > CO2 > CH4 gas composition17. This difference may be attributed to the differences of basement lithologies between the two basins where brine-rock interaction may have produced different gas species in each case. In summary, a survey of five U deposits in the Athabasca basins shows that gas composition of fluid inclusions are promising indicators of proximity to high-grade ores and may provide valuable information about the nature of brine-rock interaction in the basement which could be at the origin of UO 2 deposition. Further investigations using 3D geological models of ore deposits and properly located samples could unravel the 3D distribution of gas species around U ores and provide invaluable information on ore-forming processes. Acknowledgements T. Lhomme and M.-C. Caumon greatly helped for Raman analyses. Samples have been provided by AREVA and CAMECO. AREVA is thanked for financial support. J. Mercadier, M. Cathelineau, M.-C. Boiron and M. Cuney have encouraged this work. D. Derome made preliminary analyses on Shea Creek and Rabbit Lake fluid inclusions. References 1. Savary V, Pagel M. The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16. Geochim Cosmochim Acta 1997;61:4479-4494. 2. Dubessy J, Pagel M, Beny JM, Christensen H, Hickel B, Kosztolanyi C, Poty B. Radiolysis evidenced by H 2-O2 and H2-bearing fluid inclusions in three uranium deposits. Geochim Cosmochim Acta 1988;52:1155-1167. 3. Derome D, Cathelineau M, Lhomme T, Cuney M. Fluid inclusion evidence of the differential migration of H2 and O2 in the McArthur River unconformity-type uranium deposit (Saskatchewan, Canada). Possible role on post-ore modifications of the host rocks. J Geochem Explor 2003;78-79:525-530. 4. Bray CJ, Spooner ETC, Longstaffe FJ. Unconformity-related uranium mineralization, McClean deposits, North Saskatchewan, Canada: Hydrogen and oxygen isotope geochemistry. Can Mineral 1988;26:249-268. 5. Richard A, Boulvais P, Mercadier J, Cathelineau M, Boiron MC, Cuney M, France-Lanord C. From evaporated seawater to uraniummineralizing brines: Isotopic and trace element study of quartz-dolomite veins in the Athabasca system. Geochim Cosmochim Acta 2013;113:3859. 6. Pascal M, Boiron MC, Ansdell K, Annesley IR, Kotzer T, Jiricka D, Cuney M. Fluids preserved in variably altered graphitic pelitic schists in the Dufferin Lake Zone, south-central Athabasca Basin, Canada: implications for graphite loss and uranium deposition. Miner Deposita 2016;51:619-636. 7. Hoeve J, Sibbald TII. On the genesis of the Rabbit Lake and other unconformity-type uranium deposits in Northern Saskatchewan, Canada. Econ Geol 1978:73,1450-1473. 8. Dargent M, Truche L, Dubessy J, Bessaque G, Marmier H. Reduction kinetics of aqueous U(VI) in acidic chloride brines to uraninite by methane, hydrogen or C-graphite under hydrothermal conditions: Implications for the genesis of unconformity-related uranium ore deposits. Geochim Cosmochim Acta 2015:167,11-26. 9. Richard A, Pettke T, Cathelineau M, Boiron MC, Mercadier J, Cuney M, Derome D. Brine-rock interaction in the Athabasca basement (McArthur River U deposit, Canada): consequences for fluid chemistry and uranium uptake. Terra Nova 2010:22,303-308. 10. Richard A, Rozsypal C, Mercadier J, Banks DA, Cuney M, Boiron MC, Cathelineau M. Giant uranium deposits formed from exceptionally uranium-rich acidic brines. Nat Geosci 2012;5:142-46. 11. Kyser TK, Cuney M. Unconformity-related uranium deposits. In (M. Cuney and K. Kyser eds) Recent and Not-So-Recent Developments in Uranium Deposits and Implications for Exploration. Mineralogical Association of Canada Short Course Series 2008:39,161-219. 12. Richard A, Cathelineau M, Boiron MC, Mercadier J, Banks DA, Cuney M. Metal-rich fluid inclusions provide new insights into unconformity-related U deposits (Athabasca Basin and Basement, Canada). Miner Deposita 2016;51:249-270. 13. Richard A, Kendrick MA, Cathelineau M. Noble gases (Ar, Kr, Xe) and halogens (Cl, Br, I) in fluid inclusions from the Athabasca Basin (Canada): Implications for unconformity-related U deposits. Precambrian Res 2014;247:110-125. 14. Richard A, Montel JM, Leborgne R, Peiffert C, Cuney M, Cathelineau M. Monazite alteration in H 2O ± HCl ± NaCl ± CaCl2 fluids at 150°C and psat: Implications for uranium deposits. Minerals 2015;5:693-706. 15. Dubessy J, Poty B, Ramboz C. Advances in C-O-H-N-S fluid geochemistry based on micro-Raman spectrometric analyses of fluid inclusions. Eur J Mineral 1989;1:517-534. 16. Mavrogenes JA, Bodnar RJ. Hydrogen movement into and out of fluid inclusions in quartz: experimental evidence and geologic implication. Geochim Cosmochim Acta 1994;58:141-148. 17. Derome D, Cuney M, Cathelineau M, Fabre C, Dubessy J, Bruneton P, Hubert A. A detailed fluid inclusion study in silicified breccias from the Kombolgie sandstones (Northern Territory, Australia): inferences for the genesis of middle-Proterozoic unconformity-type uranium deposits. J Geochem Explor 2003;80:259-275.