Hydrothermal and supergene clays of the Oklo natural reactors: conditions of radionuclide release, migration and retention

Chemical Geology 157 Ž1999. 155–174

Hydrothermal and supergene clays of the Oklo natural reactors: conditions of radionuclide release, migration and retention L. Pourcelot ) , F. Gauthier-Lafaye UMR 7517, Centre de Geochimie de la Surface, CNRS 1, rue Blessig F-67084 Strasbourg Cedex, France ´ Received 27 January 1998; revised 10 November 1998; accepted 10 November 1998

Abstract Mineralogical, chemical and O-isotopic analyses were performed on the clay matrices surrounding two natural fission reactors of the Oklo deposit. In the deeply preserved reactor 10 Ž450 m., clays are composed of hydrothermal Mg-chlorites and sudoites ŽAl-chlorites.. The wide range of their O-isotopic compositions Žfrom 0.0 to 15.1‰ SMOW. is consistent with crystallisation during conditions of criticality, when the reactors created a thermal gradient of 1008Crm. The temperature in the core of reactor is calculated to have been approximately 4008C. In contrast, in the shallow reactor 9 Ž120 m deep., vermiculitized Al-chlorites are the products of the weathering of the hydrothermal chlorites. These vermiculitized Al-chlorites are 18 O enriched Ž d18 O s 18‰ SMOW. and underwent interaction with surface fluids Žy2 to y5‰.. 149 Smr147Sm isotopic ratios of the clays of reactor 9 show a wide dispersion Ž2–3 m. of the fissiogenic Sm around the reactor. The migration of the fissiogenic Sm, which is usually retained in the deep reactors, is here related to the circulation of meteoric water under oxidising conditions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Oklo; Nuclear waste; Clays; Weathering; O-isotopes; Fission products

1. Introduction The natural nuclear fission reactors located in the uranium deposits of Gabon constitute an unique opportunity to study the long term behaviour of fission products and actinides in a very stable geological system which has remained unfolded and unmetamorphosed for 2000 Ma. Since the discovery of the first reactor in 1972, 15 new reactor zones have been recognised in the Oklo open pit and in the deeper Oklo–Okelobondo mine Ž200–600 m deep.. In the ´ reactors, the uraninite, natural spent nuclear fuel, is embedded in clay. For this reason, the Oklo reactors )

Corresponding author. Fax: q33-388-367-235; e-mail: [email protected]

provide an opportunity to study the behaviour of fission products and actinides in clays similar to back fill clays around a nuclear waste container of spent nuclear fuel. Two billion years ago, the fission reactions induced a flow of hot water which was responsible for intense alteration of the host sandstone. A major effect of this alteration was the dissolution of detrital quartz, and the migration of silica from the reactor. Thus, from a mineralogical point of view, natural reactors are characterised by two main facies from which the detrital quartz has been almost totally removed ŽGauthier-Lafaye and Weber, 1989; Gauthier-Lafaye et al., 1989.: Ž1. the core of the reactor ŽU content ranging between 20 and 60%. in which the fission reaction took place and which is com-

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 9 4 - 6

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L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

posed of uraninite grains embedded in a clay matrix; Ž2. the clayey gangue surrounding the core of the reactor, which is made of hydrothermal clays mainly composed of new crystallised chlorites. The conditions for the preservation of the reactors and the effects of the surface waterrrock interaction have never been examined. In the present study, mineralogical, chemical and O-isotope compositions of chlorites from the reactors are used to distinguish two types of Oklo reactors: the deep well preserved reactor Žreactor 10. and the shallow, weathered reactor Žreactor 9.. This comparison demonstrates that the behaviour of fission products is widely dependent on the degree of weathering of Oklo natural reactors ŽPourcelot, 1997..

2. Geological setting and sample description The geological setting of the natural fission reactors has been reviewed ŽGauthier-Lafaye et al., 1996..

The reactors are located in the Oklo–Okelobondo ´ ŽFig. 1. and Bangombe´ U ore deposits, in the 2.2 Ga old Francevillian sedimentary series, ŽBonhomme et al., 1982; Bros et al., 1993.. This series lies on Archean basement and has been subdivided into five formations named, from the bottom to the top, FA to FE ŽWeber, 1968.. The FA formation is composed of fluvial and deltaic sediments Žconglomerates and sandstones.. The thickness of this formation, which contains the Francevillian U mineralisation, ranges from 100 to 1000 m. The FA formation is overlain by the organic and pyrite-rich FB black shale formation, whose thickness ranges from 400 to 1000 m. Finally, the Francevillian sedimentation ended with the deposition of the FC, FD and FE formations which correspond to volcanoclastics Žtuff, cinerites., chert and dolomite layers interbedded into black shales. The Franceville basin has never been highly metamorphosed. Diagenesis occurred during burial at a maximum depth of 4000 m. This lead to the crys-

Fig. 1. Map and idealized cross-section of the Oklo–Okelobondo deposits showing the location of the various reactors. ´

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

tallisation of diagenetic clays Žmostly 1 Md illites and chlorites., as well as silicification of the sandstones ŽGauthier-Lafaye, 1986.. SmrNd isochrons of two authigenic clay fractions from the FB formation yield ages of 2099 " 115 Ma and 2036 " 79 Ma which are considered to be the age of the early diagenesis. Fluid inclusion studies in quartz overgrowths indicate that the maximum temperature reached during diagenesis was 180 " 208C at a pressure of 400 bars ŽOpenshaw et al., 1978; GauthierLafaye et al., 1989.. At this temperature, the sediments have reached the ‘oil window’ and petroleum migrated from the FB source rock to the FA sandstone reservoir ŽGauthier-Lafaye and Weber, 1989; Cortial et al., 1990.. Microthermometric measurements of fluid inclusions located in calcite-filling fractures and associated with the uranium mineralisation provide evidence that the mineralised fluid reached temperatures of 140 " 208C under a pressure

157

of 300 bars ŽGauthier-Lafaye and Weber, 1989.. This shows that mineralisation occurred during the uplift of the basin, when reduced fluids related to hydrocarbon migration met the oxidised uraniumbearing fluids migrating into the FA reservoir ŽGauthier-Lafaye and Weber, 1989.. This uplift is also responsible for hydrofracturing of FA sandstones. High-grade uranium ores and the reactors are often located in such hydrofractured zones ŽGauthierLafaye, 1986.. Criticality occurred in the uranium deposits when four conditions were achieved ŽCowan, 1976; Naudet, 1991; Oversby, 1996.: Ž1. high U content to ensure the critical mass of the reactor ŽU content ranging between 10 and 15%rm3 of sandstone., Ž2. high concentration of fissile 235 U isotope in the natural nuclear fuel which was achieved 2.0 Ga ago, the 235 Ur238 U being 0.037 as compared with 0.0072 at present, Ž3. low chemical content of elements which are ‘poisons’ for neutrons ŽB, V and

Fig. 2. Location of the samples from reactor 10: Gallery D73, boreholes D73-S1 and SF84.

158

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

REE nuclides., Ž4. the presence of water and organic matter in the sediments acting as moderators for the neutron flux. Both the temperature of the environment and the porosity and fractures of the rock controlled the density of the water, i.e., the reactivity of the reactor. Later during the diagenetic history of the Franceville sediments and after the uranium mineralisation event, dolerite dikes intruded the Franceville basin. This dike network reflects an important extensional event which occurred 755 " 83 Ma ago ŽSere, ` 1997.. One of these dikes, 10 m thick, cross-cuts the Oklo ore deposit. At Oklo, two main types of reactors can be described depending on their state of preservation. The deepest Oklo reactors which are located far from the dolerite dike Žmore than 50 m. have remained

shielded from any major fluid interactions since the end of criticality Ž2.0 Ga ago.. This type of reactor is represented by reactor 10 ŽFig. 1.. On the other hand, in the shallowest reactors, mainly the small ones Žreactors 7, 8 and 9., it has been shown that uranium and many fission products ŽREE, Ru, Cd, Mo, Ag. were mobilised and not retained in the uraninite as is the case for reactor 10 ŽGauthierLafaye et al., 1996.. Reactor 9, located in the Oklo open pit represents this type of reactor. We will demonstrate that the differences between these two types of reactors are mainly due to their different states of weathering. Reactor 10 is 450 m deep and is up to 30 m wide, 27 m long and 0.5 m in its maximum thickness ŽFig. 2.. In this reactor, the uranium-rich core and the hydrothermal clays are well developed. Fractures

Fig. 3. Cross-section of the reactor 9 and location of the samples from outcrops 299B, 305, 336.

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

159

affecting this reactor are filled with quartz, calcite, organic matter and sulphides. Uranium also occurs associated with chlorites in the fractures ŽEberly et al., 1995.. Hydrothermal apatites have been identified in the core and in the clays ŽGauthier-Lafaye and Weber, 1988.. Bitumen, as brittle solid organic matter, is an important phase which occurs in the clays, as well as in the core. Analysed samples are from a gallery, which cross-cuts the reactor ŽD73., and from three bore holes ŽD73-S1, SF84 and SF29. ŽFig. 2.. Sample SF29 Ž82.42 m. is of particular interest because it has an important 235 U enrichment Ž235 Ur238 U s 0.010. in newly crystallised hydrothermal chlorite, indicating that small amounts of 239 Pu were incorporated in the structure of this chlorite during or soon after criticality ŽBros et al., 1993.. Reactor 9 is located in the Oklo open pit at a depth of 120 m. It is a small Ž12 m long and 7 m wide. and highly fractured pocket of uranium ore ŽGauthier-Lafaye, 1979. characterised by high organic matter content ŽFig. 3.. The core is thin Ž10–15 cm. and the clayey gangue Ž30 cm. is not well developed and is poorly consolidated. According to Naudet Ž1991., the low U-content of the core Žless than 30%. cannot justify either the criticality of this reactor or the 235 U depletion Ž235 Ur238 U s 0.0065.. During criticality, the hydrothermal fluids passed through the fracture network allowing the formation of new crystallised hydrothermal clays up to 3 m from the core of the reactor, even above the ‘normal’, unaffected sandstones Žexample: sample GL 2823 located below impermeable FB shales.. Around the reactor, two types of fractures are observed: Ž1. fractures filled with quartz and Ž2. empty fractures, devoid of calcite. Quartz from type 1 fractures were sampled for O-isotope analysis. Samples from this reactor were collected at outcrops 299B, 305 and 336 ŽFig. 3. in the core and in the surrounding clays and sandstones.

XRD patterns were collected from oriented air dried specimens and also from the same specimens treated with ethylene glycol, hydrazine and heated at 4908C for 4 h. Relative abundances of illite and chlorite were estimated according to the intensity of the 10 ˚ peaks ŽLarque´ and Weber, 1978.. We also and 7 A used sodium citrate treatment followed with Ca saturation according to the method of Tamura Ž1957.. Electron microprobe analyses were performed on both illite and chlorite with a Cameca SX50. ICP chemical analyses of clay fractions involved dissolution with glycerin–hydrochloric acid after fusion with lithium tetraborate ŽSamuel et al., 1985.. ICPAES chemical analyses were carried out using an ARL-1400 for SiO 2 , Al 2 O 3 , K 2 O, MgO, CaO, FeO, Mn 3 O4 and TiO 2 and Corning Flame photometer for K 2 O and Na 2 O. Precise mineralogical compositions of the clay fractions were calculated on the basis of both their K contents and using the chemical compositions of the illites obtained by electron microprobe analyses. Remaining cations were assumed to relate to chemical variations of chlorites. Structural formulae of chlorites were calculated on the basis of 28 cations ŽFoster, 1962.. Samarium isotopic ratios 149 Smr147 Sm, free of any isobaric mass interferences, were measured using VG plasmaquad ICPMS. Analytical errors of both ICP instruments are better than "10%. Oxygen has been extracted from approximately 10 mg of material after overnight reaction with BrF5 at 6008C and purified and reduced to CO 2 by platinized graphite ŽClayton and Mayeda, 1966.. The d18 O values of CO 2 were measured on a VG Optima mass spectrometer with a working standard calibrated against SMOW. The mean d18 O of the last 10 oxygen isotopic analyses of NBS 28 quartz standard performed in the laboratory give 6.61‰ relative to SMOW. Analytical precision of d18 O is estimated at "0.2‰.

3. Methods

4. Mineralogical data

The - 2 mm fraction of selected samples was separated by gravity settling. Randomly and oriented materials were examined by X-ray diffraction ŽXRD. by counting 18 2 u for 1 s using Cu-K a radiation.

XRD data for the clay fractions Ž- 2 mm. from reactors 10 and 9 are compiled in Table 1. In both cases clay fractions of the host FB pelites and FA sandstones contain illite, chlorite and quartz. Chlorite

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

160

Table 1 XRD data of randomly oriented clayed fractions from reactors 10 and 9 Samples

Distance to the reactor Žm.

Faciesa

Illiter chlorite Ž%.

Chlorite 001r002

002r003

dŽ001. when ˚. heated ŽA

dŽ060. b ˚. ŽA

Other minerals c

Reactor 10 Borehole SF84 18.8 3.9 18 3.1 16.2 1.3 15.86 1.00 15.9 0.96 15.7 0.8 15.65 0.75 15.5 0.6 15.34 0.44 15.3 0.4 15.24 0.34 15.15 0.25 15.03 0.13 14.93 0.03

Pelite Pelite Pelite Pelite Pelite Pelite Pelite Pelite Pelite Cl. R. Cl. R. Cl. R. Cl. R. Cl. R.

85 75 80 0 60 65 55 0 0 0 0 0 0 0

– – – 0.5 0.5 0.6 0.6 1.1 0.5 0.5 0.5 0.5 0.6 1

– – – 0.8 0.7 0.8 0.7 1 1.4 1.5 1.4 2.1 1.9 1.6

– – – 14 14 14 14 14 14 14 14 14 14 14

1.54 1.54 1.51 1.51 1.51 1.51 1.51 1.51 1.53 1.53 1.53 1.53 1.53 1.53

Q Q Q Q Q Q Q – – – – – – –

Outcrop D73 14 17 20 26 27 28 29 30 38

1.38 0.31 0.77 0.54 0.61 0.61 2.22 0.84 3.06

Cl. R. Sand. Co. R. Cl. R. Cl. R. Cl. R. Cl. R. Pelite Sand.

0 0 0 0 0 0 0 55 0

0.3 0.4 1 0.6 0.5 0.4 0.5 0.4 0.4

3.4 4 1.2 1.9 1.5 1.3 0.9 0.8 3.4

14 14 14 14 14 14 14 14 14

1.54 1.54 1.53 1.53 1.53 1.53 1.54 n.d. n.d.

– – – – – – Q Kaol., Q Q

Borehole D73-S1 6 0.10 9 0 13 0.04 22 0.84 23 0.89 30 1.34

Sand. Co. R. Cl. R. Cl. R. Cl. R. Cl. R.

0 0 0 0 0 0

0.6 0.5 0.6 0.5 0.5 0.9

2 2.1 1.9 1.9 1.8 0.8

14 14 14 14 14 14

1.53 1.53 1.53 1.53 1.53 1.53

– – – – – –

Reactor 9 Outcrop 336 3451

Sand.

20

0.2

4.6

13.8

–

Q

Outcrop 299B 2764a 2764b 2765 2783 2786 2787 2798 2805 2808 2811

Sand. Sand. Sand. Cl. R. Cl. R. Cl. R. Cl. R. Sand. Sand. Cl. R.

86 99 86 78 42 24 75 92 97 83

0.2 2.2 0.1 1.7 0.7 0.2 0.5 2.2 3.8 3.5

4.4 0.3 5.2 0.9 3.2 8.6 2.3 1.6 0.4 0.5

14 14 13.8 12.6 12.5 13 13.5 12 12.3 12.8

– – – 1.54 – – – – – 1.49

Q Q Q Kaol., Q

Q Q, Ur. Q. Ur., Coff.

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

161

Table 1 Žcontinued. Samples

Distance to the reactor Žm.

Faciesa

Illiter chlorite Ž%.

Chlorite 001r002

002r003

dŽ001. when ˚. heated ŽA

dŽ060. b ˚. ŽA

Other minerals c

Outcrop 299B 2813 2823 2826

Cl. R. Cl. R. Sand.

59 55 31

2 0.3 0.3

0.8 2.1 3.1

13 13.8 14

1.49 1.51 1.52

Q Q Q

Outcrop 305 3109 3111 3113 3119 3120 3123 3125 3131 3139 3170

Sand. Cl. R. Cl. R. Cl. R. Cl. R. Cl. R. Cl. R. Cl. R. Sand. Sand.

22 42 46 71 85 73 96 63 100 98

0.4 0.9 0.8 1.2 2 1.2 1.4 0.9 – 0.7

1.2 1.3 1.5 1.1 0.8 1 1 1.2 – 0.5

14 13.5 13.5 13.5 12.9 13.2 12.5 13.4 – 14

– 1.51 1.51 1.51 – 1.51 – 1.51 – –

Q Kaol., Ur.

Kaol. Q, Ur.

a

Cl. R.s Clays of the reactor; Co. R.s Core of the reactor; Sand.s Sandstone. dŽ060. is obtained from bulk XRD analyses of the clay fractions. c Kaol.s kaolinite; Q s quartz; Ur.s uraninite; Coff.s coffinite. b

is the major clay mineral of reactor 10 whereas, illite is present in various proportions in the clays of reactor 9, together with small amounts of kaolinite. Chlorites of reactor 10 exhibit strong mineralogical variations which are closely related to the facies of the rock Ži.e., hydrothermal clays of the reactor vs. pelite. and also to the distance from the reactor. Chlorites located over one meter from the reactor have low I Ž001.rI Ž002. ratios Ž0.1–0.3. and I Ž002.rI Ž003. ratios ) 1. They belong to the iron ˚ . chlorite group trioctahedral Ž dŽ060. s 1.54 A ŽBrown, 1995.. Closer to the reactor, clays of the pelites contain dioctahedral chlorites Ž dŽ060. reach˚ . which exhibit constant I Ž001.rI Ž002. ing 1.51 A ratios and I Ž002.rI Ž003. ratios equal to 0.5–0.6 and 0.7–0.8, respectively. This type of chlorite has been identified as sudoite which corresponds to an Al-rich chlorite, first described at Oklo by Gauthier-Lafaye et al. Ž1996.. In the close vicinity of the reactor Žlower than 0.5 m., chlorites with I Ž001.rI Ž002. ratios higher than 1.5 belong to the Mg-chlorites group. Thus, mineralogical investigations carried out on chlorites of reactor 10 reveal the presence of an Al-chlorite halo at distances between 0.5 and 1 m from the core of the reactor and Mg-chlorites closer to the reactor. Such Al- and Mg-chlorites are usually

associated with hydrothermal processes and reflect alteration of the host sandstone of reactor 10 during criticality. ˚ clay is observed in fractured In reactor 9, a 14-A sandstones and in the clays of the reactor. This clay ˚ at ambient temexhibits a strong Ž001. peak Ž14 A perature.; furthermore, this peak shifts toward 13.8– ˚ when heated at 4908C. XRD patterns do not 12.5 A exhibit any change after ethylene glycerol solvation. This clay is dioctahedral because the dŽ060. peak ˚ The lack of position varies between 1.49 and 1.54 A. glycerol swelling and the shift of the Ž001. peak ˚ upon K or Ca saturation and heating towards 10 A are characteristic of vermiculites. The shift of the basal peak when heated is due to dehydration of interlayer water. In the present study, the shift never ˚ In order to remove the presumably exceeded 12 A. interlayer Al-hydroxides which may prevent the basal spacing of vermiculites from completely shifting, the ˚ phase clay fractions were leached enriched 14 A with sodium citrate according to the procedure of Tamura Ž1957.. This treatment was followed by cationic exchange of interlayer cations with Ca2q. After this treatment, there was no change in the XRD ˚ peak is consistent with patterns. Thus, the 14-A random chloritervermiculite interstratification.

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162

Vermiculitization of chlorites is often described in soils which are exposed to weakly acid leaching conditions; whereas, under strongly acid conditions, chlorite may be completely dissolved ŽRighi and Meunier, 1995.. Processes involving vermiculitization include the removal of the hydroxide sheet of the chlorite which is replaced by exchangeable cations and water ŽJohnson, 1964.. Furthermore, kaolinite observed together with vermiculitized chlorites may constitute the ultimate weathering product of the chlorites of the reactor ŽHerbillon and Makumbi, 1975.. Thus, vermiculitization of chlorites at the subsurface reactor 9 is probably due to supergene weathering conditions.

5. Chemical analysis of chlorites 5.1. Reactor 10 Chemical analyses of the chlorites of reactor 10 are summarised in Table 2. The chlorites located between 0.7 and 1.0 m from the core of the reactor Ži.e., in pelites surrounding the reactor. which were identified as sudoite exhibit slight chemical variations ŽFig. 4.. They are Al- and also Si-rich species ŽAl 2 O 3 s 33–34 wt.%, SiO 2 s 35–36 wt.%.. On the other hand, in the vicinity of the reactor Ž D - 0.7 m., chlorites have a low Si content ŽSiO 2 s 30–33 wt.%. and integrate various amounts of Mg and Fe

Table 2 Reactor 10: chemical compositions of chlorites Sample

% Chloritea Ž% illite, % quartz.

SiO 2

Al 2 O 3

Fe 2 O 3

Borehole SF84 17.45 52 Ž22, 26. 15.9 68 Ž10, 22. 15.86 75 Ž25, 0. 15.7 50 Ž25, 25. 15.65 55 Ž25, 20. 15.5 100 15.34 100 15.3 100 15.24 100 15.15 100 15.03 100 14.93 100

32.8 35.19 36.6 35.42 35.43 34.47 30.25 30.61 32.92 30.76 32.39 33.7

28.2 34.51 34.33 33.05 33.79 34.14 28.83 27.51 33.06 25.16 25.22 26.6

17.60 1.29 3.58 1.55 1.32 1.22 1.49 6.16 1.76 9.97 4.48 4.86

Outcrop D73 14 100 20 100 17 70 Ž25, 5. 26 100 27 100 28 100 29 100 30 75 Ž0, 25. 38 55 Ž40, 5.

34.5 33.9 31.0 31.4 31.0 31.1 34.4 34.3 33.5

25.3 28.3 23.6 25.8 27.0 28.2 33.7 35.9 34.4

– – – – – – – – –

Borehole D73-S1 6 100 9 100 13 100 22 100 23 100 30 100

31.8 30.6 31.7 30.4 30.5 35.7

24.7 22.9 23.5 25.1 25.3 33.8

– – – – – –

a

FeO

MgO

H 2O

Ý

FerŽFe q Mg.

10.21 12.36 11.78 11.64 11.76 11.11 21.09 21.9 14.36 19.53 15.53 12.47

11.15 14.76 13.71 16.41 15.87 15.91 13.46 13.82 15.36 14.57 15.44 15.14

100 100 100 100 100 100 100 100 100 100 100 100

0.46 0.11 0.13 0.13 0.12 0.17 0.14 0.12 0.13 0.20 0.28 0.34

17.9 14.2 22.9 14.3 10.6 9.7 5.3 3.7 13.3

7.7 6.3 9.2 12.6 16.1 18.3 12.7 12.2 2.9

13.3 14.4 13.3 14.9 14.3 13.4 13.8 13.8 15.9

98.7 97.1 100 99 99 100.7 99.9 99.9 100

0.54 0.53 0.55 0.36 0.25 0.21 0.17 0.13 0.70

9.1 8.9 7.5 7.0 6.7 3.3

16.6 17.2 19.7 18.6 18.3 11.9

15.3 15.4 15.9 15.1 15.5 15.1

97.5 95 98.3 96.2 96.3 99.8

0.21 0.20 0.16 0.15 0.15 0.12

0.0 1.89 0.0 1.93 1.84 3.14 4.89 0.0 2.54 0.0 6.94 7.23

Chlorite content calculated from chemical composition of the clay fraction.

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163

Fig. 4. Chemical variations of chlorites of reactors 10 and 9.

in the octahedral sheets ŽFerŽFe q Mg. s 1.8–3.. From the core of the reactor to the edge the Mg-content of the chlorites decreases; whereas, iron and vacancies slightly increase. Thus, chemical compositions of the chlorites of reactor 10 are related to the distance from the core of the reactor. According to Hillier and Velde Ž1991., temperature has a strong effect on the chemical compositions of chlorites. The use of chlorites as a potential

geothermometer is widely discussed in the literature ŽCathelineau and Nieva, 1985; Walshe, 1986; Kranidiotis and Mac Lean, 1987; Cathelineau, 1988; De Caritat et al., 1993; Essene and Peacor, 1995.. The chlorite geothermometer of Cathelineau Ž1988., based on the AlŽIV. content of the tetrahedral sites, is empirically calibrated between 150 and 3008C. It does not take into account the influence of rock chemistry ŽKranidiotis and Mac Lean, 1987; Jowet,

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164

1991; Xie et al., 1997.. Temperatures of crystallisation, calculated for samples in which chlorite was the only detected mineral vary widely between 310 ŽSF84 15.34. and 1708C ŽD73-S1 30. ŽTable 3.. At the contact with the reactor, calculated temperatures range from 155 to 2208C and progressively increase to 240–3108C at 0.4 m from the reactor ŽFig. 5.. Finally, between 0.5 and 2 m from the core of the reactor, the temperature slowly decreases to 150– 1708C. These results are in good agreement with a previous study on oxygen isotopes of chlorites ŽGauthier-Lafaye et al., 1989., which has shown that chlorites located next to the core of the reactor crystallised during its cooling stage when neutron bombardment had stopped. In spite of the dispersion of the data points in Fig. 5, we can propose that the highest temperatures recorded in samples located further than 0.4 m allow us to estimate a maximum geothermal gradient around the reactor. These data

Table 3 Reactor 10: calculated temperatures of crystallisation of chlorites using geothermometer of Cathelineau Ž1988. Sample

Distance to the reactor Žm.

AlŽIV.

T Ž8C. a

points fit on a line corresponding to a geothermal gradient of 1008Crm which, when extrapolated to the core of the reactor Ždistance 0 m., gives a maximum temperature of 3808C. 5.2. Reactor 9

Borehole SF84 15.5 0.6 15.34 0.44 15.3 0.4 15.24 0.34 15.15 0.25 15.03 0.13 14.93 0.03

0.78 1.15 1.13 0.92 1.06 0.84 0.73

189 308 302 235 278 207 175

Outcrop D73 14 20 29 26 27 28

2 0 0.73 0.46 0.73 0.73

0.68 0.69 0.86 0.93 1.02 1.10

156 159 214 238 266 293

Borehole D73-S1 6 0.1 9 0 13 0.04 22 0.84 23 0.89 30 1.34

0.87 0.88 0.89 0.98 0.96 0.72

218 221 225 252 247 171

a

Fig. 5. Calculated temperatures of crystallisation of the chlorites of the reactor 10 using geothermometer of Cathelineau Ž1988. as a function of the distance to the reactor.

T sy61.92q321.98 AlŽIV. ŽCathelineau, 1988..

In the surface weathered reactor 9, chemical analyses were completed on both the illites and chlorites of the sandstones and clays ŽTable 4.. Illites exhibit slight variations in their chemical compositions. However, the potassium content Žbetween K 2 O s 4.16 and 5.17 wt.%., as well as the sum of the components of illites Žbetween Ý s 86.58 and 91.0%., for the hydrothermal clays of the reactor are lower than the illites in the sandstone ŽK 2 O s 8.22– 9.14% and Ý s 90.4–94.35%, respectively.. Lower K content of illites is often associated with H 3 Oq substitution in the interlayer sites ŽNewman and Brown, 1987.. Vermiculitized chlorites of reactor 9 are usually Si- and Al-rich Ž30 to 38 wt.% and 30 to 37 wt.%, respectively. and Mg– and Fe–depleted Ž10 to 16 wt.% and 2 to 7 wt.%, respectively. compared with common chlorites of the sandstones. Furthermore, the sum of the cations of the vermiculitized chlorites is lower than for common chlorites. Low cation and high Al-contents ŽFig. 4. are as-

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165

Table 4 Reactor 9: chemical compositions of clays Electron microprobe analyses of illites Samplea

SiO 2

Al 2 O 3

FeO

Outcrop 299B GL 2764 Ž6. GL 2765 Ž6. GL 2783 Ž18. GL 2796 Ž4. GL 2823 Ž1.

49.41 40.67 45.09 47.45 47.18

33.61 28.94 33.20 35.3 36.26

Outcrop 305 GL 3120 Ž10. GL 3123 Ž11. GL 3139 Ž12. GL 3170 Ž4.

44.07 44.49 47.90 47.5

MgO

K 2O

Na 2 O

CaO

TiO 2

Ý Ž%.

1.40 11.11 2.20 1.27 0.57

0.92 4.15 0.59 0.42 0.40

8.32 5.05 5.73 6.17 7.37

0.40 0.17 0.19 0.04 0.37

0.16 0.13 0.12 0.34 0.02

0.14 0.47 0.01 0.02 0.00

94.35 90.68 87.02 91 92.17

30.90 30.34 32.13 33.77

6.54 5.49 0.60 0.87

0.81 0.86 1.37 0.55

4.16 5.17 8.22 9.14

0.13 0.13 0.04 0.47

0.19 0.18 0.12 0.1

0.00 0.02 0.05 0.01

86.58 86.68 90.4 92.38

% Chloriteb Ž% illite, % quartz.

SiO 2

Al 2 O 3

FeO

MgO

K 2O

Na 2 O

CaO

TiO 2

Outcrop 336 GL 3451

65 Ž10, 25.

30.44

25.7

26.39

6.62

–

–

–

Outcrop 299B GL 2764 Ž3. GL 2783 Ž9. GL 2786 GL 2787 GL 2798 GL 2811 GL 2813 GL 2823 GL 2823 Ž6. GL 2826

– – 85 Ž10, 5. 85 Ž10, 5. 50 Ž45, 5. 80 Ž15, 5. 85 Ž10, 5. 65 Ž30, 5. 65 Ž30, 5. 75 Ž15, 10.

26.74 38.86 39.10 38.26 32.24 36.72 36.07 28.82 36.47 29.97

25.65 31.18 37.05 35.19 36.19 34.07 34.55 30.86 30.76 28.07

24.19 2.93 3.77 5.67 6.88 10.45 10.66 16.89 9.18 19.4

9.09 0.90 0.72 0.79 5.77 1.47 2.14 10.88 7.59 5.18

0.57 0.05 – – – – – – 0.58 –

0.01 0.02 – – – – – – 0.05 –

Outcrop 305 GL 3109 GL 3109 Ž6. GL 3111 GL 3111 Ž15. GL 3113 GL 3119 GL 3120 Ž9. GL 3125

70 Ž10, 20. 70 Ž20, 10. 90 Ž10, 0. 90 Ž10, 0. 90 Ž10, 0. 75 Ž25, 0. – 50 Ž50, 0.

38.87 39.60 34.51 30.67 33.33 34.79 38.04 34.17

30.00 31.79 31.43 22.9 29.62 31.23 31.69 32.00

5.87 2.94 13.26 13.15 15.06 13.52 12.10 16.88

10.26 9.04 5.34 6.36 6.38 4.18 2.63 2.03

– 0.57 – 0.03 – – 0.05 –

– 0.08 – 0.04 – – 0.02 –

Chemical compositions of chlorites Samplea

a b

H 2O

Ý Ž%.

–

10.85

100

0.06 0.15 – – – – – – 0.11 –

0.02 0.01 – – – – – – 0.21 –

– – 19.35 20.09 18.93 17.28 16.58 12.56 – 17.38

86.33 74.11 100 100 100 100 100 100 84.94 100

– 0.25 – 0.31 – – 0.58 –

– 0.04 – 0.01 – – 0.01 –

15.01 – 15.46 – 15.6 16.29 – 14.93

100 84.3 100 73.48 100 100 85.1 100

Samples in italic s electron microprobe analysis, number of analyses between parenthesesrnormal style s whole ICP-AES analysis. Chlorite content calculated from ICP-AES chemical compositions of the clay fractions.

sumed to be due to major hydration which may ˚ peak when explain the observed shift of the 14 A heated. Leaching of Mg and Fe due to vermiculitization of chlorite during surface weathering has already been described by Proust et al. Ž1986.. Thus,

chemical data point out the importance of the weathering process for the formation of the vermiculitized chlorites and that these chlorites have different origins compared with the high temperature Mg-chlorites and sudoites of the deeper reactor 10.

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

166

6. O-isotope data The range of the O-isotope compositions of the clays of the Franceville basin sediments ŽGauthierLafaye et al., 1989. and of the reactors is large: between 0 and 18‰ ŽFig. 6.. Such a large range reflects various physico-chemical conditions of crystallisation which occurred Ž1. during the diagenesis of the Francevillian sediments, Ž2. during the criticality, in rocks surrounding the reactors, and Ž3. in clays which were affected by recent weathering. 6.1. Oxygen isotopes in diagenetic minerals Microthermometric measurements on fluid inclusions in diagenetic quartz overgrowths and O-isotope compositions of syngenetic illites and chlorites allowed us to determine the maximum temperature of the diagenetic fluids and its O-isotope composition

ŽGauthier-Lafaye et al., 1989.. Maximum temperature during diagenesis, before the uranium mineralisation event, has been measured at 180 " 208C. Calculations to determine the O-isotope compositions of the corresponding water were carried out using the mineral–water fractionation factors of Eslinger and Savin Ž1973. for illite and of Mingchou Ž1984. for chlorite. Results give a d18 O of the water ranging between 5.0 to 8.0‰. Slightly lower temperatures of 140 " 208C were recorded in calcite fluid inclusions associated with the uranium mineralisation. The calculated d18 O of the water in isotopic equilibrium with the calcite has been estimated at 6.5 " 1.5‰ ŽGauthier-Lafaye et al., 1989.. In this study, only minerals cogenetic with the uranium mineralisation event and the fission reactions were analysed. Therefore, in the following, diagenetic water refers to the water of the mineralisation stage whose mean d18 O value is 6.5‰.

Fig. 6. O-isotope compositions of the diagenetic, newly crystallised clays and of the clays of the reactors 10 and 9.

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

6.2. Oxygen isotopes in clays and quartz of the reactors Hydrothermal alteration due to criticality has been detected in chlorites of reactor 10 and in quartz of reactor 9. In reactor 10, d18 O of the hydrothermal chlorites are highly variable ranging from 0.0 ŽSF29

167

82.42. to 12.8‰ ŽSF84 18.00. ŽTable 5.. Against the core of the reactor, Mg-chlorites are 18 O enriched Žbetween 7.1 and 11.7‰.; whereas, between 0.4 and 0.8 m from the core, they are surprisingly very 18 O depleted Žbetween 1.8 and 0.0‰. and further from the reactor, in the pelite zone Žfrom 0.8 to 1.4 m., sudoites are 18 O enriched Žbetween 11.8 and 15.1‰..

Table 5 Reactor 10: measured and corrected O-isotope compositions and calculated temperatures of equilibrium of the chlorites Faciesa

d 18 Ocorrected Ž‰. c

T Ž8C. d

T Ž8C. e

12.8 12.5 12.3 5.9 6.4 9.6 8.9 7.1 1.3 1.8 5.6 5.4 10.0 11.7

– – 15.1 3.2 5.4 7.8 7.1 7.1 1.3 1.8 5.6 5.4 10.0 11.7

– – 155 540 415 320 345 345 640 595 405 415 255 195

– – 150–200 510–770 400–580 315–430 335–465 335–465 595– 550– 375–535 350–500 225–305 185–250

1.38 2.22 0.77 0.31 0.54 0.61 0.61 0.84 3.06

11.8 9.6 11.2 8.0 7.1 1.0 2.4 3.7 12.5

11.8 9.5 11.2 8.0 7.1 1.0 2.4 1.0 14.2

195 240 205 280 310 670 545 640 150

185–250 230–315 195–260 275–375 295–415 615– 510– 615– 145–195

0.10 0.00 0.04 0.84 0.88 1.34 0.80

7.1 8.4 8.0 5.7 5.9 5.1 0

7.1 8.4 8.0 5.7 5.9 5.1 0

310 270 280 360 355 385 ) 700

300–420 260–365 270–300 350–495 340–485 365–530 795–

Mineralogy of chlorites Žcontent %. b

Distance to the reactor Žm.

d

Borehole SF84 18.8 Pelite 18 Pelite 16.2 Pelite 15.9 Pelite 15.86 Pelite 15.7 Pelite 15.65 Pelite 15.5 Pelite 15.34 Pelite 15.3 Cl. R. 15.24 Cl. R. 15.15 Cl. R. 15.03 Cl. R. 14.93 Cl. R.

Fe-chlorite Ž20%. Fe-chlorite Ž25%. Sudoite Ž30%. Sudoite Ž68%. Sudoite Ž75%. Sudoite Ž50%. Sudoite Ž55%. Sudoite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%.

3.9 3.2 1.4 1.1 1.0 0.9 0.8 0.7 0.5 0.4 0.34 0.3 0.2 0.1

Outcrop D73 14 Cl. R. 17 Sand. 20 Cl. R. 26 Cl. R. 27 Cl. R. 28 Cl. R. 29 Cl. R. 30 Pelite 38 Sand.

Fe-chlorite Ž100%. Fe-chlorite Ž70%. Fe-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Sudoite Ž100%. Sudoite Ž75%. Sudoite Ž55%.

Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Mg-chlorite Ž100%. Sudoite Ž100%. Sudoite Ž100%.

Sample

Borehole D73-S1 6 Sand. 9 Co. R. 13 Cl. R. 22 Cl. R. 23 Cl. R. 30 Cl. R. SF29 Cl. R. Ž82.42 m. a

18

O Ž‰.

Cl. R.s Clays of the Reactor; Co. R.s Core of the Reactor; Sand.s Sandstone. Chlorite content calculated from chemical composition of the clay fraction. c 18 d Odetrital quartzs 10‰rd18 Oillites 13.8‰ ŽGauthier-Lafaye et al., 1989.. d Temperatures calculated using fractionation calculated using the bond approach of Savin and Lee Ž1988. and assuming d18 Owa ter s 6.5‰. e Temperatures calculated using fractionation calculated using the bond approach of Savin and Lee Ž1988. and assuming d18 Owa ter ranges between 6 and 9‰. b

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

168

In the detrital quartz grains of the host sandstones of reactor 9, d18 O values exhibit slight variations Ž9.9 to 11.3‰. ŽTable 6.. On the other hand, d18 O values of the quartz-filling fractures are highly variable Ž10.1 to 17.0‰.: they increase with the distance from the reactor ŽFig. 8.. It is assumed that during criticality, and because water acted as an important moderator for neutrons, that the waterrrock ratio was high and that dissolution and crystallisation of minerals during fission reaction operation did not significantly change the oxygen isotopic composition of the diagenetic water. Therefore, the temperatures of crystallisation of the clays during fission reaction operations were calculated assuming a constant isotopic composition of the diagenetic fluid. However, fission reactions ceased when the development of newly formed clay minerals affected the permeability of the rocks surrounding the reactor and therefore decreased the reactivity of the fission reactions. At that stage, the core of the reactor and the surrounding rock can be considered as a quasi closed system with a low waterrrock ratio. We will see that such a system may have affected the oxygen isotopic composition of the water located close to the core of the reactors. The temperature of oxygen isotope equilibrium between chlorite and water is calculated using the bond-type approach of Savin and Lee Ž1988.. Assuming a d18 O value of 6.5‰ for the water, the temperature range obtained for chlorites of the gangue

is wide: 150 ŽD73 38. to 6708C ŽD73 28. ŽTable 5.. The highest temperatures are obtained for chlorites located at a distance ranging from 0.4 to 0.8 m from the core of the reactor. Taking into account the analytical errors Ž"0.2‰. on the d18 O measurement, the precision on the calculated temperatures yield "158C for temperatures ranging between 200 and 4008C. From the core of the reactor to its margin, the temperature first increases and then decreases ŽTable 5 and Fig. 7.. One has to note that the lowest temperatures are for clays located inside or against the core of the reactor. The chlorites located too close to the neutron bombardment source could not have crystallised during the criticality but during the cooling stage of the reactor. Seven chlorites Žnot plotted on Fig. 7. located between 0.4 and 0.8 m from the core of the reactor exhibit very low d18 O values Ž- 4‰. and temperatures calculated with the oxygen isotopic composition of the diagenetic fluids Ž6.5‰. are higher than 4508C ŽTable 5.. Such values are unrealistic, chlorite being unstable at such high temperatures ŽLiou et al., 1974.. One of these chlorites ŽSF29—82.42 m. yields an important 235 U enrichment which has been interpreted as resulting from the decay of 239 Pu which migrated from the core of the reactor during the end stage of the fission operation or soon after fission reactions ceased ŽBros et al., 1993.. Such a sample provides evidence that fluids have percolated through the core of the reactor

Table 6 Reactor 9: O-isotope compositions and temperatures of the quartz 18

Sample

Facies

Distance to the reactor Žm.

d

GL 2764 GL 2774 GL 2778 GL 2826 GL 3170 GL 2712 GL 2719 GL 2779 GL 3109 GL 3172 GL 3446 GL 3447

Detrital quartz Detrital quartz Detrital quartz Detrital quartz Detrital quartz Quartz fracture Quartz fracture Quartz fracture Quartz fracture Quartz fracture Quartz fracture Quartz fracture

6.0 2.3 0.3 2.5 1.2 3.3 2.0 1.2 1.0 2.0 2.0 2.2

10.1 9.9 11.3 10.3 11.2 17.0 16.0 10.1 13.4 16.1 16.0 15.8

a b

O Ž‰.

T Ž8C. a

T Ž8C. b

– – – – – 220 240 420 300 240 240 245

– – – – – 260 280 480 350 280 280 290

Temperature calculated with quartz–water fractionation from Clayton et al. Ž1972., assuming d18 Owa ter s 6.5‰. Temperature calculated with quartz–water fractionation from Bottinga and Javoy Ž1973., assuming d18 Owa ter s 6.5‰.

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

Fig. 7. Temperatures of isotopic equilibrium of the chlorites as a function of the distance to the reactor 10.

169

which is for higher temperatures Ž500–8008C.. In the 200–5008C range, the curve from Clayton et al. Ž1972. is also in good agreement with that obtained by Matsuhisa et al. Ž1979. using the three-isotope method at 2508C. For comparison, temperatures of quartz crystallisation are given using both types of equation: low ŽClayton et al., 1972. and high ŽBottinga and Javoy, 1973. temperature equations ŽTable 6.. Between 1 and 3.5 m from the core of the reactor, the calculated temperatures of the quartz filling the fractures range between 420 and 2208C ŽFig. 8.. These temperatures increase by 508C when using the fractionation equation of Bottinga and Javoy Ž1973.. These results allow us to estimate a very high thermal gradient around the reactor of 1008Crm. This also demonstrates that during criticality, fractures were important pathways for the fluids and that they should have played an important role in fission products migration. 6.3. Oxygen isotopes in weathered clays

and have altered the uraninites allowing the mobilisation of fissiogenic Pu. Because this occurred at the end of the fission operation, when the system was almost closed, we suggest that the oxygen isotope composition of the water was sensitive to the H 2 O– UO 2 interaction. As the uraninites are highly 18 O depleted minerals Ž y18‰ . ŽPourcelot and Gauthier-Lafaye, 1998., the diagenetic water having reacted with these uraninites should have also been 18 O depleted allowing the crystallisation of the seven chlorites with very low d18 O. For these samples Ž0.4 to 0.8 m from the core., temperatures calculated using the diagenetic water Ž6.5‰. are therefore unrealistic and possess no geological meaning. Below 4008C, the various quartz–H 2 O fractionation curves commonly given in the literature are not concordant. This is because experimental data are difficult to obtain at such low temperatures, Žthe rate of exchange of oxygen isotopes between quartz and water is very slow. and therefore, the fractionation curves have low precision or are estimated from theoretical calculations. In this study, we have preferred to use the fractionation equation given by Clayton et al. Ž1972. which has been established for the temperature range between 200 and 5008C rather than the equation of Bottinga and Javoy Ž1973.

Vermiculitization of chlorites of reactor 9 has been attributed to surface weathering. These chlorites are strongly 18 O enriched Žbetween 13.8 and 17.2‰. compared with non-vermiculitized chlorites Žbetween 7.9 and 11.2‰. ŽTable 7. and with the hydrothermal chlorites of the reactor 10. In addition to vermiculi-

Fig. 8. O-isotope compositions and temperatures of crystallisation of the quartz-filling fractures as a function of the distance to the core of the reactor 9.

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

170

Table 7 Reactor 9: O-isotope compositions of the chlorites Samples

Faciesa

Mineralogical compositionb

d

Ft 336 GL 3451

Sand.

Chlorite Ž65%., illite Ž10%., quartz Ž25%.

10.8

299B GL 2786 GL 2787 GL 2798 GL 2811 GL 2813 GL 2823 GL 2826

Cl. R. Cl. R. Cl. R. Cl. R. Cl. R. Clay Sand.

Vermiculitized chlorite Ž85%., quartz Ž5%., illite Ž10%. Vermiculitized chlorite Ž85%., quartz Ž10%., illite Ž5%. Vermiculitized chlorite Ž50%., illite Ž45%., quartz Ž5%. Vermiculitized chlorite Ž80%., illite Ž15%., quartz Ž5%. Vermiculitized chlorite Ž85%., illite Ž10%., quartz Ž5%. Chlorite Ž65%., illite Ž30%., quartz Ž5%. Chlorite Ž75%., illite Ž15%., quartz Ž10%.

17.2 17.2 14.3 17.1 16.4 7.9 11.2

Ft 305 GL 3109 GL 3111 GL 3113 GL 3119 GL 3125 GL 3135

Sand. Cl. R. Cl. R. Cl. R. Cl. R. Cl. R.

Chlorite Ž70%., illite Ž10%., quartz Ž20%. Vermiculitized chlorite Ž90%., illite Ž10%. Vermiculitized chlorite Ž90%., illite Ž10%. Vermiculitized chlorite Ž75%., illite Ž25%. Vermiculitized chlorite Ž50%., illite Ž50%. Vermiculitized chlorite Ž5%., illite Ž95%.

10.8 13.8 15.0 15.9 17.1 10.8

a b

18

O Ž‰.

Cl. R.s Clays of the Reactor; Co. R.s Core of the Reactor; Sand.s Sandstone. Chlorite content calculated from ICP-AES chemical compositions of the clay fractions.

tized chlorites, most of the clay fractions contain various proportions of quartz and illite. In order to determine the oxygen isotope composition of the pure vermiculitized chlorite phase, the d18 O of the samples containing less than 5% of quartz are plotted on the Fig. 9a against their proportion of illite. This figure shows that the d18 O value increases with increasing proportion of vermiculitized chlorite. By extrapolation, the d18 O value of pure vermiculitized chlorite can be estimated to be approximately 18‰. The fractionation factor between a ‘mean’ vermiculitized chlorite ŽGL 2813. ŽSi 3.5 Al 0.5 O 10 . ŽAl 2.9 Fe 0.7Mg 1.0 .ŽOH. 8 and water is calculated using the bond-type approach of Savin and Lee Ž1988.: 10 3 ln a Vermiculitized chlorite – waters7.887 Ž 10 3 . Ty1 q 1.284 Ž 10 6 . Ty2 y 0.148 Ž 10 9 . Ty3 q 0.013 Ž 10 12 . Ty4 y 14.033

d18 O of the water in equilibrium with vermiculitized chlorite Ž d18 O s 18‰. is plotted vs. T in Fig. 9b. Temperature of 135 " 158C is deduced for vermiculitized chlorite Ž d18 O s 18‰. being in equilibrium with the diagenetic fluid Ž d18 O s 6.5 " 1.5‰.. This is not in agreement with the temperature at the

time of fission reactions. Thus, based on the mineralogical and chemical data, it is more likely that the vermiculitized chlorite equilibrated with present day surface waters Žy2 to y5‰, according to Louvat et al. Ž1995. at a temperature ranging between 25 and 508C ŽFig. 9b...

7. Implications for fission products migration 7.1. During criticality It is now well established that fission products and actinides have migrated Ž1. during criticality, Ž2. at the time of dolerite dike intrusion, Ž1.0 billion years after criticality. and Ž3. more recently, due to supergene weathering ŽNaudet, 1991; Hidaka et al., 1992; Bros et al., 1994.. During criticality, the uranium ores which surrounded the area where the fission reactions occurred were significantly transformed due to the action of hydrothermal fluids. The main changes were: Ž1. the migration of silica out of the sandstones and the net inflow of Mg, Fe and Al leading to the formation of newly crystallised clays ŽTchibena-Makosso, 1982., and Ž2. the formation of

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

171

tered during reactor operation. When the uranium content of the rocks surrounding the area where fission reaction occurred reached the critical mass, then the fission reaction began allowing the reactor to grow and move. During that stage, and during the cooling of the reactors, in addition to major elements ŽSi, Al, Fe, Mg, K and U., many trace elements were mobilised, such as the volatile fission products ŽCd, Cs. and noble gases ŽXe, Kr. and the alkalines ŽRb, Cs. and alkaline earth elements ŽBa, Sr. ŽJaneczeck and Ewing, 1992; Hidaka et al., 1994; GauthierLafaye et al., 1996; Hidaka et al., 1998.. These elements escaped the reactor zone, as they were not significantly trapped in the uraninites and in the associated clays or other newly crystallised minerals Žphosphates, carbonates.. However, most of the clays surrounding the reactors contain small amounts of fissiogenic elements, mainly REE ŽBros et al., 1993.. In one sample ŽSF29 82.42., enriched 235 U Žthe decay product of 239 Pu. has been found. This reflects the mobility of 239 Pu at the time of criticality or soon afterward, during the cooling stage Žhalf life of 239 Pu is 23,400 years.. All of these clays have very low oxygen isotope compositions, reflecting that they have crystallised at high temperatures Ž300–4008C., during the fission reactions operation. 7.2. During weathering

Fig. 9. Reactor 9. Ža. Variation of the O-isotope composition of the clays with their proportion of vermiculitized chlorites and illites. Žb. O-isotope compositions of the fluid in isotopic equilibrium with the vermiculitized chlorites as a function of the temperature. Ž1. after Gauthier-Lafaye et al. Ž1989.; Ž2. after Louvat et al. Ž1995..

a 10 to 40 cm thick layer with very high uranium content. Two mechanisms resulted in the formation of the high uranium content layer: Ž1. the loss of silica which results in an increase of the relative uranium content; Ž2. the influx of uranium due to the hydrothermal fluids. In reactor 2, 80% of the Si migrated from the reactor and 50% of uranium en-

In the upper part of the Oklo deposit, the clays of the shallow reactor 9 Ž120 m deep. record weathering conditions. Such an environment may explain the loss of portions of the fissiogenic elements ŽLREE, Ru, Pd, Mo, Ag and Cd. from the reactor zone ŽCurtis et al., 1989; Hidaka et al., 1992.. O-isotope compositions of the clays and their 149 Smr147 Sm ratios are good tracers of the water–rock interactions and of the behaviour of fission products under weathering conditions, respectively ŽFig. 10.. The 18 O enriched clay fractions Žover 13‰. reflect isotopic exchange between chlorites and surface fluids during vermiculitization. The 147 Sm and 149 Sm rare earth are fissiogenic nuclides: the fission of a hundred nuclei of 235 U yields 2.283 147 Sm and 1.064 149 Sm, respectively ŽEngland and Rider, 1988.. Depending on their cross-sections, isotopes of these fissiogenic REE are transmuted by neutron capture. Because the cross-section of 149 Sm Ž s s 82,000

172

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

Fig. 10. 149 Smr147 Sm ratios and d18 O of the clay fractions of the samples near reactor 9.

barns. is much larger than that of 147 Sm Ž s s 154 barns., the 149 Smr147 Sm ratio is low in the core of the reactor Žclose to 0. compared with 0.920 for the natural value. Therefore, a 149 Smr147 Sm ratio lower than 0.920 in a sample located around the core provides evidence of migration of fissiogenic Sm. However, because the Sm content is not known, this anomaly does not give a precise idea of the exact proportion of fissiogenic Sm in the sample. Fig. 10 shows that from the edges to the core of the reactor the Sm anomaly increases. This strongly suggests that the fissiogenic Sm has dispersed from the core towards the edges of the reactor. Furthermore, this contamination by fissiogenic Sm is closely related with the 18 O enrichment of the clays ŽFig. 10.. This confirms that the surface water percolated through the core of the reactor and dissolved uraninite, probably under oxidising conditions, releasing their fission products and dispersing the fissiogenic LREE. Some of these fission products were then trapped by the clays which crystallised during this weathering process.

8. Conclusion Mineralogical, chemical and isotopic investigations were completed on the clays and quartz of the reactors 10 and 9 in order to determine the tempera-

tures related to the criticality event and to evaluate the extent to which weathering alteration has affected the shallowest reactors. The hydrothermal chlorites, which crystallised during criticality or soon afterward during the cooling of the reactor, show a wide range of oxygen isotopic compositions due to the steep thermal gradient around the core of the reactor Ž1008Crm.. This high gradient is consistent with the results of models of the heat and fluid transfer in the host rock during criticality which takes into account the permeability and the thermal conductivity of the various rocks, the thermal power of the reactors Ždue to 235 U and 239 Pu fissions. and the duration of the fission reactions ŽRoyer et al., 1995.. In the reactor located at only 120 m depth, weathering has had an important effect on the behaviour of the fission products. Oxidation of the pyrites by the surface water increased its acidity when it percolated through the pyrite-rich FB black shales. This water migrated through the fracture network and reached the core of the reactor, resulting in partial dissolution of uraninite and the dispersion of the fission products in the surrounding rocks. At the macroscopic scale, such weathering has no effect on the hydrothermal clays which have kept their original structure. However, weathering allowed the vermiculitization of the chlorite and oxygen isotope exchange with present meteoric waters. This surface acidic water is responsible for the dissolution of the calcite originally

L. Pourcelot, F. Gauthier-Lafayer Chemical Geology 157 (1999) 155–174

present in the fractures and of the hydroxyapatites which are always present in the deeper reactors.

Acknowledgements We would like to thank J.-J. Frey, R. Rouault, J. Samuel and R. Wendling for help and technical assistance. This paper benefited from comments by P. Stille, G. Shields and R.C. Ewing. Critical comments and reviews by S. Sheppard and H. Hidaka have greatly helped to improve this manuscript. This work was supported by the European programme ‘Oklo-Natural Analogue, Phase 2’ Žcontract CCE FI4W-CT96-0020.. [PD]

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