Radiolytic (H2, O2) and other Trace Gases (CO2, CH4, C2H6, N2) in Fluid Inclusions from Unconformity-related U Deposits

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Radiolytic (H2, O2) and other Trace Gases (CO2, CH4,

C2H6, N2) in Fluid Inclusions from

Unconformity-related U Deposits

Antonin Richard

To cite this version:

Antonin Richard. Radiolytic (H2, O2) and other Trace Gases (CO2, CH4, C2H6, N2) in Fluid

Inclusions from Unconformity-related U Deposits. Procedia Earth and Planetary Science, Elsevier,

2017, 17, pp.273-276. �10.1016/j.proeps.2016.12.053�. �hal-02437143�

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

Available online at www.sciencedirect.com

Peer-review under responsibility of the organizing committee of WRI-15 doi: 10.1016/j.proeps.2016.12.053

ScienceDirect

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

Radiolytic (H

2

, O

2

) and other trace gases (CO

2

, CH

4

, C

2

H

6

, N

2

) in

fluid inclusions from unconformity-related U deposits

Antonin Richard

a,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.

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 ore 1-3

. 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 detected 3

. 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: antonin.richard@univ-lorraine.fr

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274 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 deposition

3,7,8

. 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 <1.75 Ga Athabasca basin unconformably overlies a 2.0-1.8 Ga basement. The current maximum thickness of the sedimentary cover (mostly sandstones and siltstones) is ~1.5 km and could have originally been as much as ~5 km. The U deposits are generally located near the basement/cover interface, and structurally controlled by sub-vertical faults rooted in graphite-rich basement meta-pelites. UO2 ores have been dated to between 1.6 and 1.4 Ga

and successive late episodes of mineralization, and/or recrystallisation occurred until ~0.7 Ga11. Sandstone silicification, and quartz and dolomite veins are spatially and temporally associated with the main clay alteration minerals and UO2 ores. Hence, they are likely to host fluid inclusions that are relicts of the mineralizing fluids. The

veins crosscut various lithologies such as meta-pelites and pegmatoids as well as basin sandstones. Veins were sampled from 4 deposits (Shea Creek, Rabbit Lake, Eagle Point, P-Patch) from various parts of the basin, in relatively fresh to extensively altered rocks. Prior to this study, fluid inclusions have been characterized petrographically and by microthermometry and similar fluid inclusions in the same samples were analysed destructively by LA-ICPMS12. Trace gases composition of fluid inclusions from McArthur River have been published previously3 and corresponding published U concentrations in fluid inclusions determined by LA-ICPMS are presented here9,10. Primary and pseudosecondary fluid inclusions show two types of 100-200°C basinal brines of evaporated-seawater origin, showing evidence for mixing: a “NaCl-rich brine” end-member (Cl>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 P-Patch: 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 O2 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 “CaCl2

-rich 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 N2 and are devoid of O2 (Fig. 1d)3.

Alteration of NH4-bearing feldspar and biotite in the basement and formation of NH4-bearing micas in the clay

alteration haloes are probably the dominant control on the N2 concentration in the fluid inclusions 6

. 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 O2 in fluid inclusions from the

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

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) Fluid type U (ppm) CO2 CH4 N2 H2 O2 C2H6

~ 10 NaCl-rich brine 1 0 15 0 85 0 0

~ 1 NaCl-rich brine 8 0 0 0 0 100 0

~ 10 NaCl-rich brine 20 0 68 0 30 0 2

~ 10 NaCl-rich brine 42 0 0 0 67 5 28

~ 0.5 CaCl2-rich brine 34 0 0 0 60 40 0

~ 1 CaCl2-rich brine 634 0 0 0 35 65 0

H2 is present in fluid inclusions from all deposits, even in samples located at more than 10 meters from

high-grade 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 metapelites 5 . 0 10 20 30 40 50 60 70 80 P-Patch Rabbit Lake Shea Creek Eagle Point H2 CH4 N2 CO2 C2H6 O2 Fr eq uency (%) Liquid Vapor Laser 10 μm (a) (c) (d) (e) (b) CO2 0 10 20 30 40 50 60 70 80 90 100 CH4 0 10 20 30 40 50 60 70 80 90 100 N2 0 10 20 30 40 50 60 70 80 90 100 H2 CH4 O2 500 1500 2500 3500 4500 Wavenumber (cm-1₎ 5000 10000 15000 20000 25000 In te n sity Detectable gases No detectable gas N = 81 (e)

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

According to other authors, CO2 and CH4 could both be the products of brine-graphite interaction 4,7

.

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 composition 17

. 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 UO2 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

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Cosmochim Acta 1997;61:4479-4494.

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