Geochemistry of palaeofluids circulation in the Franceville basin and around Oklo natural nuclear reaction zones (Gabon)

Journal of Geochemical Exploration 69–70 (2000) 245–249 www.elsevier.nl/locate/jgeoexp

Geochemistry of palaeofluids circulation in the Franceville basin and around Oklo natural nuclear reaction zones (Gabon) R. Mathieu*, M. Cuney, M. Cathelineau CREGU-UMR G2R 7566 Ge´ologie et Gestion des Ressources Mine´rales et Energe´tiques, Universite´ Henri Poincare´, BP239, F-54506 Vandoeuvre-le`s-Nancy Cedex, France

Abstract The geochemical characteristics of five palaeofluids in the Franceville basin (Gabon) have been studied using a multidisciplinary investigation of fluid inclusions: (1) a low saline meteoric water firstly migrating through the basement and injected into the basin, (2) high chlorine brines equilibrated with carbonate and evaporitic layers in sandstones, (3) hydrocarbon-rich fluid derived from organic matter maturation in the pelites, (4) a mineralising fluid (Lwmix) originating from the mixing of (1) and (2), (5) a hot fluid related to the nuclear reactors functioning. The interaction of fluids (3) and (4) is responsible for the U-mineralisation at the redox boundary developed between sandstones and pelites. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: diagenesis; fractures; uranium; geochemistry; fluid inclusions; Oklo

1. Introduction For the past 25 years, all microthermometric studies around Oklo reaction zones (Franceville basin, southeastern part of Gabon, Fig. 1) have focussed mainly on the detection of high temperature fluids. These fluids are related to the functioning of the natural nuclear reactors that have induced a strong disturbance in the host sandstones. A bibliographical synthesis of the geological history of the Franceville basin and the nuclear reactor zones has allowed one to recognise several types of fluid circulations (Michaud and Mathieu, 1998). In relation to two main diagenetic phases,there are four major tectono-metamorphic events and one supergene alteration phase. Models for the formation of U deposits from the Franceville basin are limited by the paucity of data * Corresponding author. Fax: ⫹ 33-3-83-91-38-01. E-mail address: [email protected] (R. Mathieu).

regarding the nature, origin and evolution of the fluids involved and their relationship to alteration and ore deposition. Few quantitative analytical data on the chemical composition of fluid inclusions or of their accompanying phases are available. These data, together with an appreciation of their significance with respect to the mineral paragenesis, are a prerequisite for a better understanding of the mineralising process. This paper presents an attempt to understand the origin of fluid production, and the eventual evolution of fluid chemistry through time, using data obtained from mineral paragenesis in veins and from a multitechnique database on fluid inclusions. An original aspect of fluid inclusion studies is the geochemical approach to fluid chemistry by combining the analysis of fluid inclusion gases, Cl, other ions and hydrocarbons content in liquid using Raman and FTIR microspectroscopies, microthermometry, Laser Ablation– Optical Emission Spectroscopy (LA–OES) and bulk crush leach analyses. This work discusses the

0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00054-6

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geochemical results of the fluid inclusion study on mineral veins collected in two different sedimentary formations (sandstones and pelites) of the Franceville basin, more particularly around Oklo reactor zones (Fig. 1), indexed FA and FB, respectively (Weber, 1969).

2. Results Structural data reflect at least three major tectonic phases (Hassenforder et al., 1994): (1) a first extensional phase related to normal faulting oriented NE– SW, E–W and NW–SE, (2) a second extensional phase related to regional flexurating responsible for major N–S faulting and (3) a compressional phase related to dextral-inverse N15⬚, sinistral-inverse N165⬚ strike-slip faults.

Fig. 1. Locations of U deposits with normal uranium isotopic ratio (U) and U deposits with natural fission reactors (RZ) of the Franceville basin (Gabon). Simplified geological map adapted from Bros (1993).

Pre-ore stage 1

Wall rocks Dis solu tio n/Overgrowths of detrit al Quartz grain s Chlo rit e 1 Ac cessories Dis solu tio n REE/P min erals Calc it e An kérit e Pyrit e Galena Chalc opyrit e Bit umen Pit chblende Oriented Recryst˚ Quartz

Ore stage 2

Post-ore stage 3

U-Pb-S ric h

Vein s Mic rocrystallin e Quartz Fib rous Chlo rit e Early milk y Quartz Rosette lik e Chlo rit e Late geodic Quartz Calc it e Dolo mit e An kérit e Pyrit e Galena Chalc opyrit e Bit umen

U-Pb-S ric h

Pit chblende Idio morphic Quartz Oriented Recryst˚ Quartz

Structural Phases1 Dir ectio ns of vein s

Majo r Min or

Pre- Exten sion al N -S Flexu re N W-SE N E-SW E-W N -S

σ

σ

1

3

N

Exten sion al N -S Flexu re

σ

3

σ

1

N

N -S N E-SW N W-SE

Com p ressive strik e-slip

σ

σ

2

3

σ

1

Fig. 2. Paragenetic mineral sequence of altered wall rocks (FB–FA sandstones and Archean basement) and veins. Line thicknesses represent schematically the relative quantities of the mineral deposited.

R. Mathieu et al. / Journal of Geochemical Exploration 69–70 (2000) 245–249

247

450

Th (˚C) 400

Stage 1

Stage 2

Stage 3

Lw h1 350

Lw h2

LwRf

Lw h2 Lw mix

300

Lw Rf Lw

Lw

Lw

250

200

Lw mix

Lwh2 150

Lwh1 Lw

100

Tm ice (˚C) 50 -65

-55

-45

-35

-25

-15

-5

0

Fig. 3. Homogenization temperature (Th) vs ice melting temperature (Tm,ice) diagram applied to aqueous fluids (Lwh1, Lwh2, Lwmix, Lw and LwRf) in inclusions (primary and secondary FIP) from veins related to pre-ore, ore and post-ore stages (1, 2 and 3) in the Franceville basin. Lwh fluids displaying Tm ice lower than H2O–NaCl eutectic temperature contain Ca and Li.

Three paragenetic stages (Fig. 2) have been correlated to these three regional structural phases. The Archean basement and the sandstone–pelite series of the Franceville basin are affected by fractures mainly filled by quartz–chlorite–calcite–sulphides and barren or U-mineralised bitumens. Stage 1 consists of overgrowths on and dissolution of detrital quartz, chloritisation of detrital feldspar grains, monazite and zircon dissolution and florencite recrystallisation in an FA sandstone matrix. Subvertical stage 1 fractures are mainly filled with chlorite and quartz and to a lesser extent with calcite, barren bitumen and Pb/ Fe/Cu sulphides. Stage 2 consists of a U-mineralised bitumen precipitating in the FA sandstone matrix. Subvertical stage 2 fractures are filled with the same minerals as in stage 1, but carbonates, U-mineralised bitumen and sulphides are the more important phases. Stage 3 (late silicification) shows approximately the same fracture fillings as stage 1. Fluid inclusions are classified according to their

host mineral and the relative chronology between the host planes. The results of this distribution are presented in a Th =Tm ice diagram (Fig. 3). During the first extensional phase, a low saline, likely to be meteoric, fluid (Lw; 2.2–8.5 wt% NaCl) has been heated in the basement (190–210⬚C, trapping temperatures obtained with pressure correction at 1 kbar, considering a lithostatic regime). This less saline fluid originated from the surface (meteoric origin) and migrated probably through the basement rocks, before it went up, along major N–S faults, into the upper levels of the FA formation. The circulation of this fluid is responsible for the quartz overgrowth and the formation of clay minerals that contributed to the closure of the primary porosity. Diagenetic low temperature (135–155⬚C) brines are high chlorine, calco-sodic ([Cl] ⬎ 6 m, from 28 wt% NaCl; Lwh2 to 30 wt% CaCl2; Lwh1) and are rich in Ca, Li, SO4 and Br (Fig. 4). These highly saline brines are equilibrated with evaporitic domains

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Fig. 5. Model of the palaeofluid circulations in relation to uranium mineralisations in the Franceville basin.

Fig. 4. (A) Na/Ca vs Na/Li mass ratios (ppm) obtained from LA– OES measurements. (B) Cl/SO4 vs Na/K molar ratios. (C) Cl/Br molar ratio vs Cl content (mmol/kg water) obtained by the crush leach analysis of representative fluid inclusions. Comparison with reference fluid compositions: a, intragranitic fluids (Michard, 1990); b, oil field brines (Carpenter et al., 1974; Kharaka et al., 1987; Fisher and Kreitler, 1987); c, Soultz fluids, Rhine graben (Pauwels et al., 1993); d, evaporites (Ayora et al., 1994); seawater evaporation trend (Fontes and Matray, 1993).

(Fig. 4). They originated from the carbonate– sulphate-cemented layers in the FA sandstones, which are mainly concentrated in the central part of the Franceville basin. During the second extensional phase, a mixing between the brines (Lwh1, Lwh2) and the meteoric water (Lw) occurred giving a moderately saline fluid (Lwmix; 155–220–C). The interaction of the mineralising fluid (Lwmix) and hydrocarbons C9 and C10-rich fluids (Lhc) derived from organic matter maturation in the FB pelites is responsible for the U-mineralisation. The low to moderate saline (3–18 wt% NaCl) fluids with higher temperatures (LwRf; 200–410⬚C) contain traces of O2, CH4 and CO2 and are related to the reactors’ functioning. During the last compressional phase, the palaeofluid circulations are mainly responsible for the precipitation of the barren calcite.

3. New mode of U deposition in the Franceville basin The low temperature diagenetic brines (Lwh), are responsible for U–Zr–P–Pb–REE leaching by the dissolution of accessory minerals (zircon and monazite) in FA sandstones (Mathieu, 1999). The Th/U ratio increase from monazite (Th/U ˆ 18.6) to the Th-silicate alteration phase (Th/U ˆ 88.7) is interpreted as a result of an alteration by the oxidising brines with leaching of U together with LREE and P. During the ore-stage in extensional regime, the fracturation has allowed the mixing of the injection of meteoric recharge (Lw) in the basin and the deep basinal brines (Lwh). The mixing fluid (Lwmix) is expelled laterally, due to the compaction of sandstones, and upwards along subvertical N–S fractures. Due to the rapid burial of the FB pelites, overpressured fluids (hydrocarbons C9 and C10-rich fluids: Lhc) occurred in the black shales during this stage. The extensional phase allowed the transmission of these overpressured fluids to the FA sandstone reservoir. The mixing of Lhc and Lwmix is responsible for the U-mineralisation at the redox boundary developed during the diagenesis between FA sandstones with oxidising conditions and hydrocarbon producing pelites (Fig. 5).

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