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Origin and genesis of the Oklo uranium ore deposits (Gabon): results from a basin scale fluid flow modelling
Journal of Geochemical Exploration 69–70 (2000) 165–171 www.elsevier.nl/locate/jgeoexp
Origin and genesis of the Oklo uranium ore deposits (Gabon): results from a basin scale fluid flow modelling J. Salas*, K. Bitzer, C. Ayora Institut Jaume Almera, CSIC, Lluis Sole´ i Sabaris, s/n, E-8028 Barcelona, Spain
Abstract Hydrologic processes that have contributed to the mineralogenesis of uranium ore deposits in the Franceville basin at Oklo (Gabon) are still uncertain. Based on stratigraphic, sedimentological and petrophysical data available from the outcrops at the Oklo area, we have performed a series of simulation experiments in order to constrain hypothetical paleoflow systems at the basin scale. Two-dimensional (2D) models have been applied to test hypotheses concerning convective flow systems in the Franceville basin and its transport capacity, the possible location of recharge and discharge areas and the existence of tectonosedimentary drains. Modelling results show that 2D flow models are insufficient to describe a system flux that is compatible with the location and distribution of the uraninite bodies in the Franceville basin. A 3D model has therefore been developed that successfully represents possible paleoflow systems with sufficient system flux in those parts of the basin, where the mineralisation is observed. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Oklo; Uranium ore deposits; Modelling; Fluid flow
1. Introduction The Oklo uranium ore deposit has gained much interest as it is the only known place where natural reactors have been preserved. The deposit is located in fluviodeltaic sandstones at the western margin of the Franceville basin (Fig. 1). Nuclear fission took place during the Paleoproterozoic, about 2 × 10 ⫺9 years ago, providing fissiogenic products and modifying the isotopic composition of some elements (Gauthier-Lafaye, 1986). The location therefore represents a natural analogue of a nuclear waste disposal site and has been intensely studied with respect to the possible migration of radionuclides. This study, however, is principally concerned with
* Corresponding author. E-mail address: [email protected] (J. Salas).
the paleoflow systems that may have contributed to the generation of the deposit (Salas et al., 1997).
2. Objectives and methodology Redox mechanisms are the main geochemical processes in the formation of uranium ores. Therefore, the possible distribution of organic matter, within the deposit and in the basin, is a principal factor controlling the geochemical processes that have contributed to the uranium precipitation. Several principal hydrogeological factors for the modelling have been analysed, such as hydraulic conductivities, fracturation and preferential flow by sedimentologic and tectonic drains. Due to the sparse information, two conceptual models have been tested: (1) thermal convection flow induced by a given geothermal gradient, and (2) topography-driven flow due to a given topographic gradient. Pressure and
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00055-8
166
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
N Libreville
Equator
Lambarene Port Gentil
Moanda Oklo Franceville
300 km Fig. 1. Geographical location of the Oklo uranium deposits, Rep of Gabon.
temperature have been used as state variables, with the fluid phase concentration as a constant. Flow modelling was performed using the code CODE_BRIGHT (Coupled Deformation, Brine Gas and Heat Transport), developed in the Departament d’Enginyeria del Terreny, UPC, Barcelone (Olivella, 1995).
3. Geological setting The Francevillian series is composed of five stratigraphic units called FA, FB, FC, FD and FE (Fig. 2). The detrital FA formation is located at the base of the log: conglomerates at the bottom, and fluviodeltaic
Fm. Lithology Stanley Pool
FE FD FC FB2
Fine grain. sandst. with kaolini. cement Chert Rhyolitic tuff Pyroclastic sandstones
FB1
Carbonated manganese protore Dolomite Black shales Sandstones Radioactive conglomerates
U
FA
Archean basement
U
Uranium mineralization
Fig. 2. Log of Francevillian series in the Franceville basin (Gauthier-Lafaye and Weber, 1989).
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
UO2(OH)2
2UO2(CO3)2
UO2CO3
UO2+ 2
-20
UO2OH
+
-10
log fO2
The Oklo uranium deposits are uraninite lenses located in a sandy unit from the top of the FA formation, in close contact with the organogenic blade pelites of the FB formation. The mineralogenesis process is a consequence of the interaction between two different redox systems: (1) oxidizing water with a high concentration of uranium aqueous species, and (2) reducing environment with disperse organic matter. The uranium precursor was a detrital uraniferous sediment crossed by the oxidizing water that incorporates uraniferous aqueous species. The uraninite precipitation was a consequence of the decrease of the oxygen fugacity due to the interaction with the reducing environment (Fig. 3). Thus, the flow system compatible with the uranium ores must consist of an oxidizing water that dissolves uraniferous minerals from basal FA sediments, and which will then interact with the organic matter placed on the top of FA formation (in contact with the FB pelitic sediments).
T=150 C Pp (CO2)=1 atm [U]=1.0e-7 M
0
-30
Fe 2
UO 2 (s) Uraninite
-40 -50
Fe 2+
[F e]
[F e]
O
=1
3 (s)
.0e
-7
=1
M
.0e
-4 M
-60 3
4
5
6
7
8
pH Fig. 3. The pH–log[f (O2)] fields of the uranium aqueous species at 150⬚C and equilibrium Fe 2⫹/hematite equilibrium line.
sandstones and pelites, at the top of the series. It is 100 m thick at the edges of the basin and 1000 m thick in the most subsident areas (Gauthier-Lafaye and Weber, 1989). The FB formation lithologies have a reductor behaviour as a consequece of disperse organic matter. They are characterised by a grain size distribution finer than FA, with thickness ranging from 400 m at the edges of the basin to 1000 m in the central areas. The FC formation contains deposits such as jaspers and dolomites and it represents the end of the sedimentary cycle that started with the sedimentation of FB. The FD formation basically consists of pelites and black shales with thin silicated ash layers and organic matter, and the FE formation is constituted of sandstones, lutites and silty black shales.
4. Thermal convection flow Thermal convection in the uranium bearing fluvialdeltaic sandstones of the FA formation has been tested in a two-dimensional (2D) W–E section, with a length of 40 km, through the Franceville basin. Steady state flow without sources and sinks and a fixed porosity distribution have been assumed. A fixed temperature of 190⬚C is assumed at the base of formation FA, and a temperature of 150⬚C is calculated for the reactor areas (Gauthier-Lafaye, 1986). Several tests with different hydraulic conductivities have been performed in order to test the onset of convection.
3000 1000 2000 2000 1000 3000 40000 Depth (m)
Depth (m)
ENE-WSW 4000 0
00
2000 2
4000 4
6000 6
167
8000 8
10000 10
12000 12
14000 14
16000 16
18000 18
20000 20
Distance Distance (m)(km) 30 50 70 90 110 130 150 170 190 T (ºC)
Sedimentary basin profile Schematic flow path Uranium ore location Fig. 4. Temperature field and convective cells generated by the thermal convection model.
168
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
Depht (m)
WSW-ENE 0
6001 3001 1 Distance (km)00 0.00 10000.0020000.0030000.0040000. 0050000.0060000.
2000
4000
6000
0
10
20
100
20
140
30
180
220
40
50
260
60
70
80
90
300 T(ºC)
Sedimentary basin profile Schematic flow path Uranium ore location Fig. 5. Temperature field and a schematic flow path generated by the first topographic-driven flow model.
Permeabilities have been compared with literature data for similar formations and from petroleum deposits. Effects of compaction and stratigraphic and tectonic drains have not been included. Convective flow takes place, for Rayleigh numbers above 40, on the basis of the basin (Fig. 4). This Rayleigh number is achieved with equivalent permeability values of 6 m/y Kx 10 m=y; Kz 4 m=y for formation FA. This value may vary depending on the equivalent permeability value considered for formation FB, which is located above formation FA. The maximum convective flow rates calculated from these values are in the order of 0.1 m 3/m 2 y. Though the set of parameters represents a realistic scenario, we have discarded free convection as a flow mechanism for two reasons: (1) several flow cells are calculated with the flow ascending at several locations of the cross-section, whereas the Oklo uranium deposits are restricted to the western margin of the basin, and (2) the flow system does not provide a reasonable conceptual model that allows the state of oxidation of the solution during the transport of uranium to be maintained.
5. Topography-driven flow Fluid flow due to the infiltration of meteoric water, and circulation in response to the paleo-topographic relief during the metallogenetic time (GauthierLafaye, 1986) has been modelled as an alternative
hypothesis. Two simplified paleo-topographic reliefs have been tested: (1) a topographic gradient affecting the whole basin and the basement exposed in its boundaries, and (2) a topographic gradient that affects only the granitic basement materials exposed on the basin flanks. In case (1) the primary uranium source is thought to be located at the base of formation FA, whereas in (2) the source of uranium would be located within the plutonic massif. The first case is simulated within a 100 km section representing the former basin fill with hydrodynamic parameters similar to the convection case and with reduced permeability for the overlying formation FB. Flow rates of 0.1 m 3/m 2 y are reached at a topographic gradient of 0.0123. This gradient was considered to be an upper limit. Considering a discharge area in the upper formations of the reactor zone provides reasonable flow patterns with an ascending flow in the western margin (Fig. 5) coinciding with the assymmetric distribution of ore bodies in the basin. The reducing effect of the lower part of formation FB would facilitate elevated uranium concentrations. However, the simulated flow system is not appropriate for the specific location of ore bodies at the Oklo site; further, meteoric fluids having passed through formation FB would have lost most of their oxygen content, whereas oxidising fluids are required to keep uranium in solution. Additionally, there is no evidence of paleotopographic elevation at the eastern margin of the basin. Therefore, we consider this flow model to be unsatisfactory.
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
Depht (m)
0
169
WSW-ENE
2000 4000 6000 8000
10000 0.000 0.00
5
10 10000.00
15
20 20000.00
25
30 30000.00
35
40
45
50
Distance (km)
25 120 160 200 240 280 320 360 400 T (ºC)
Sedimentary basin profile Squematic flow paths Faults Uranium ore location Fig. 6. Temperature field and schematic flow paths generated by the second topographic-driven flow model.
The second case involves a paleotopographic gradient of 0.06 at the western margin of the basin, principally affecting the basement material and to a lesser degree the sedimentary. A conductive fault zone representing the Le´you-Mounana fault is included and it acts as a drainage (Figs. 6 and 7). The simulated fluid flow rates are not sufficient, and the characteristics of the flow field do not coincide with the location of uranium ores. The fault zone alters the flow system only locally; geochemically, the redox state obtained with the Fesilicates, plagioclase and K-feldspars dissolution is too low to explain uraninite dissolution and the transport of uranium aqueous species (water in equilibrium with these minerals is found in the stability field of uraninite). 6. Three-dimensional topography-driven flow Due to the deficiencies of the 2D models, we applied a three-dimensional (3D) flow model that is more suited to represent flow focussing. The model includes a set of 9512 nodes and 8176 quadrilateral elements, representing the major tectonosedimentary structures that may have influenced regional flow. The following features are included (Fig. 7): 1. A granitic block with low permeabilities located at the border of the basin. The tensor of hydraulic
conductivities is assumed to be anisotropic with its main component being vertical. 2. The Le´you-Mounana fault as a regional tectonic system that controls sedimentation in the margin between the Lastourville basin and the Franceville basin. 3. Three different units within formation FA characterized by different hydrodynamic properties and their position with respect to the Le´you-Mounana fault. One of these units represents the basal high permeable conglomerates of unit FA. 4. A transversal topographic gradient (WSW–ENE) affecting the plutonic massif and superficial sediments, and a longitudinal regional topographic gradient (NNW–SSE), which is the principal source for topography-driven flow. In order to maintain the oxidant redox state in the fluids before reaching the location of the Oklo ore body, recharge must have taken place through the formation FA sandstones cropping out in the Lastourville basin. The hydraulic connection between formation FA sandstones in the Lastourville basin and higher conglomeratic units of formation FA in the Franceville basin is provided through the Le´youMounana fault. The resulting flow system is mainly parallel to the longitudinal axis of the basin with preferential pathways at the base of formation FA. Flow velocities are
170
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
Fig. 7. Geometrical schema of the conceptual 3D topography-driven flow model: (a) the basin as a whole, and (b) enlargement of the lower series.
Fig. 8. Flow vectors in the marginal FA conglomerates (in the Oklo area).
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171 ⫺8
171
in the order of 3–8 × 10 m/s with maximum values in the marginal conglomerates. Flow in the plutonic massif is in the order of 10 ⫺10 –10 ⫺12 m/s, several orders of magnitude below. Regional flow is mainly descending and parallel to the base of formation FA. Due to the geometry of formation FA and the assumed characteristics of the Le´you-Mounana fault, the flow system turns on the footwall block into the E–W direction, and finally turns into a vertically ascending flow in the Oklo area (Fig. 8), where hydrocarbons were stored. In this way, the flow system obtained explains the interaction of the oxidizing water with the basal FA sediments and with the reducing fluids in the Oklo area. The 3D model is able to explain the location of uranium ore at the western margin of the Franceville basin as a result of the tectonic structures of the basin and the presence of highly permeable units of formation FA at the basin margin.
flow required paleotopographic gradients, which are poorly supported by geologic data and do not fit well with the geochemical system. The proposed 3D model is capable of reproducing flow patterns that are in accordance with our data. Furthermore, it permits us to explain oxidising solutions with uranium aqueous species in the upper part of the marginal conglomerates of formation FA, where the main flow component is vertical.
7. Conclusions
Gauthier-Lafaye, F., 1986. Les gisements d’uranium du Gabon et les re´acteurs d’Oklo. Mode`le me´talloge´nique de gıˆtes a fortes teneurs du Prote´rozoı¨que infe´rieur: Me´moires Sc. Ge´ologique, 78, Strasbourg, pp. 3–192. Gauthier-Lafaye, F., Weber, F., 1989. The Francevillian (Lower Proterozoic) uranium ore deposits of Gabon. Economic Geology 84, 2267–2285. Olivella, S., 1995. CODE_BRIGHT Manual (not published). Dpt. de Ing. del Terreno—UPC, Barcelona. Salas, J., Ayora, C. Modeling flow and reactive transport at basin scale: 1st joint EC-CEA workshop of the Oklo II working group, Sitges, June 1997.
It has been shown that 2D models are insufficient to explain the generation and location of uranium ores at the Oklo site in the Franceville basin. Thermal convection is not capable of providing sufficient fluid flow and solute for the precipitation of ores. Furthermore, the locations of ore precipitation derived from the thermal convection model do not correspond with the actual location at Oklo. Topography-driven
Acknowledgements This work forms part of the Oklo Phase II project, funded by the EC contract FI4W-CT96-0020, and by ENRESA.
References
Origin and genesis of the Oklo uranium ore deposits (Gabon): results from a basin scale fluid flow modelling J. Salas*, K. Bitzer, C. Ayora Institut Jaume Almera, CSIC, Lluis Sole´ i Sabaris, s/n, E-8028 Barcelona, Spain
Abstract Hydrologic processes that have contributed to the mineralogenesis of uranium ore deposits in the Franceville basin at Oklo (Gabon) are still uncertain. Based on stratigraphic, sedimentological and petrophysical data available from the outcrops at the Oklo area, we have performed a series of simulation experiments in order to constrain hypothetical paleoflow systems at the basin scale. Two-dimensional (2D) models have been applied to test hypotheses concerning convective flow systems in the Franceville basin and its transport capacity, the possible location of recharge and discharge areas and the existence of tectonosedimentary drains. Modelling results show that 2D flow models are insufficient to describe a system flux that is compatible with the location and distribution of the uraninite bodies in the Franceville basin. A 3D model has therefore been developed that successfully represents possible paleoflow systems with sufficient system flux in those parts of the basin, where the mineralisation is observed. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Oklo; Uranium ore deposits; Modelling; Fluid flow
1. Introduction The Oklo uranium ore deposit has gained much interest as it is the only known place where natural reactors have been preserved. The deposit is located in fluviodeltaic sandstones at the western margin of the Franceville basin (Fig. 1). Nuclear fission took place during the Paleoproterozoic, about 2 × 10 ⫺9 years ago, providing fissiogenic products and modifying the isotopic composition of some elements (Gauthier-Lafaye, 1986). The location therefore represents a natural analogue of a nuclear waste disposal site and has been intensely studied with respect to the possible migration of radionuclides. This study, however, is principally concerned with
* Corresponding author. E-mail address: [email protected] (J. Salas).
the paleoflow systems that may have contributed to the generation of the deposit (Salas et al., 1997).
2. Objectives and methodology Redox mechanisms are the main geochemical processes in the formation of uranium ores. Therefore, the possible distribution of organic matter, within the deposit and in the basin, is a principal factor controlling the geochemical processes that have contributed to the uranium precipitation. Several principal hydrogeological factors for the modelling have been analysed, such as hydraulic conductivities, fracturation and preferential flow by sedimentologic and tectonic drains. Due to the sparse information, two conceptual models have been tested: (1) thermal convection flow induced by a given geothermal gradient, and (2) topography-driven flow due to a given topographic gradient. Pressure and
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00055-8
166
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
N Libreville
Equator
Lambarene Port Gentil
Moanda Oklo Franceville
300 km Fig. 1. Geographical location of the Oklo uranium deposits, Rep of Gabon.
temperature have been used as state variables, with the fluid phase concentration as a constant. Flow modelling was performed using the code CODE_BRIGHT (Coupled Deformation, Brine Gas and Heat Transport), developed in the Departament d’Enginyeria del Terreny, UPC, Barcelone (Olivella, 1995).
3. Geological setting The Francevillian series is composed of five stratigraphic units called FA, FB, FC, FD and FE (Fig. 2). The detrital FA formation is located at the base of the log: conglomerates at the bottom, and fluviodeltaic
Fm. Lithology Stanley Pool
FE FD FC FB2
Fine grain. sandst. with kaolini. cement Chert Rhyolitic tuff Pyroclastic sandstones
FB1
Carbonated manganese protore Dolomite Black shales Sandstones Radioactive conglomerates
U
FA
Archean basement
U
Uranium mineralization
Fig. 2. Log of Francevillian series in the Franceville basin (Gauthier-Lafaye and Weber, 1989).
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
UO2(OH)2
2UO2(CO3)2
UO2CO3
UO2+ 2
-20
UO2OH
+
-10
log fO2
The Oklo uranium deposits are uraninite lenses located in a sandy unit from the top of the FA formation, in close contact with the organogenic blade pelites of the FB formation. The mineralogenesis process is a consequence of the interaction between two different redox systems: (1) oxidizing water with a high concentration of uranium aqueous species, and (2) reducing environment with disperse organic matter. The uranium precursor was a detrital uraniferous sediment crossed by the oxidizing water that incorporates uraniferous aqueous species. The uraninite precipitation was a consequence of the decrease of the oxygen fugacity due to the interaction with the reducing environment (Fig. 3). Thus, the flow system compatible with the uranium ores must consist of an oxidizing water that dissolves uraniferous minerals from basal FA sediments, and which will then interact with the organic matter placed on the top of FA formation (in contact with the FB pelitic sediments).
T=150 C Pp (CO2)=1 atm [U]=1.0e-7 M
0
-30
Fe 2
UO 2 (s) Uraninite
-40 -50
Fe 2+
[F e]
[F e]
O
=1
3 (s)
.0e
-7
=1
M
.0e
-4 M
-60 3
4
5
6
7
8
pH Fig. 3. The pH–log[f (O2)] fields of the uranium aqueous species at 150⬚C and equilibrium Fe 2⫹/hematite equilibrium line.
sandstones and pelites, at the top of the series. It is 100 m thick at the edges of the basin and 1000 m thick in the most subsident areas (Gauthier-Lafaye and Weber, 1989). The FB formation lithologies have a reductor behaviour as a consequece of disperse organic matter. They are characterised by a grain size distribution finer than FA, with thickness ranging from 400 m at the edges of the basin to 1000 m in the central areas. The FC formation contains deposits such as jaspers and dolomites and it represents the end of the sedimentary cycle that started with the sedimentation of FB. The FD formation basically consists of pelites and black shales with thin silicated ash layers and organic matter, and the FE formation is constituted of sandstones, lutites and silty black shales.
4. Thermal convection flow Thermal convection in the uranium bearing fluvialdeltaic sandstones of the FA formation has been tested in a two-dimensional (2D) W–E section, with a length of 40 km, through the Franceville basin. Steady state flow without sources and sinks and a fixed porosity distribution have been assumed. A fixed temperature of 190⬚C is assumed at the base of formation FA, and a temperature of 150⬚C is calculated for the reactor areas (Gauthier-Lafaye, 1986). Several tests with different hydraulic conductivities have been performed in order to test the onset of convection.
3000 1000 2000 2000 1000 3000 40000 Depth (m)
Depth (m)
ENE-WSW 4000 0
00
2000 2
4000 4
6000 6
167
8000 8
10000 10
12000 12
14000 14
16000 16
18000 18
20000 20
Distance Distance (m)(km) 30 50 70 90 110 130 150 170 190 T (ºC)
Sedimentary basin profile Schematic flow path Uranium ore location Fig. 4. Temperature field and convective cells generated by the thermal convection model.
168
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
Depht (m)
WSW-ENE 0
6001 3001 1 Distance (km)00 0.00 10000.0020000.0030000.0040000. 0050000.0060000.
2000
4000
6000
0
10
20
100
20
140
30
180
220
40
50
260
60
70
80
90
300 T(ºC)
Sedimentary basin profile Schematic flow path Uranium ore location Fig. 5. Temperature field and a schematic flow path generated by the first topographic-driven flow model.
Permeabilities have been compared with literature data for similar formations and from petroleum deposits. Effects of compaction and stratigraphic and tectonic drains have not been included. Convective flow takes place, for Rayleigh numbers above 40, on the basis of the basin (Fig. 4). This Rayleigh number is achieved with equivalent permeability values of 6 m/y Kx 10 m=y; Kz 4 m=y for formation FA. This value may vary depending on the equivalent permeability value considered for formation FB, which is located above formation FA. The maximum convective flow rates calculated from these values are in the order of 0.1 m 3/m 2 y. Though the set of parameters represents a realistic scenario, we have discarded free convection as a flow mechanism for two reasons: (1) several flow cells are calculated with the flow ascending at several locations of the cross-section, whereas the Oklo uranium deposits are restricted to the western margin of the basin, and (2) the flow system does not provide a reasonable conceptual model that allows the state of oxidation of the solution during the transport of uranium to be maintained.
5. Topography-driven flow Fluid flow due to the infiltration of meteoric water, and circulation in response to the paleo-topographic relief during the metallogenetic time (GauthierLafaye, 1986) has been modelled as an alternative
hypothesis. Two simplified paleo-topographic reliefs have been tested: (1) a topographic gradient affecting the whole basin and the basement exposed in its boundaries, and (2) a topographic gradient that affects only the granitic basement materials exposed on the basin flanks. In case (1) the primary uranium source is thought to be located at the base of formation FA, whereas in (2) the source of uranium would be located within the plutonic massif. The first case is simulated within a 100 km section representing the former basin fill with hydrodynamic parameters similar to the convection case and with reduced permeability for the overlying formation FB. Flow rates of 0.1 m 3/m 2 y are reached at a topographic gradient of 0.0123. This gradient was considered to be an upper limit. Considering a discharge area in the upper formations of the reactor zone provides reasonable flow patterns with an ascending flow in the western margin (Fig. 5) coinciding with the assymmetric distribution of ore bodies in the basin. The reducing effect of the lower part of formation FB would facilitate elevated uranium concentrations. However, the simulated flow system is not appropriate for the specific location of ore bodies at the Oklo site; further, meteoric fluids having passed through formation FB would have lost most of their oxygen content, whereas oxidising fluids are required to keep uranium in solution. Additionally, there is no evidence of paleotopographic elevation at the eastern margin of the basin. Therefore, we consider this flow model to be unsatisfactory.
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
Depht (m)
0
169
WSW-ENE
2000 4000 6000 8000
10000 0.000 0.00
5
10 10000.00
15
20 20000.00
25
30 30000.00
35
40
45
50
Distance (km)
25 120 160 200 240 280 320 360 400 T (ºC)
Sedimentary basin profile Squematic flow paths Faults Uranium ore location Fig. 6. Temperature field and schematic flow paths generated by the second topographic-driven flow model.
The second case involves a paleotopographic gradient of 0.06 at the western margin of the basin, principally affecting the basement material and to a lesser degree the sedimentary. A conductive fault zone representing the Le´you-Mounana fault is included and it acts as a drainage (Figs. 6 and 7). The simulated fluid flow rates are not sufficient, and the characteristics of the flow field do not coincide with the location of uranium ores. The fault zone alters the flow system only locally; geochemically, the redox state obtained with the Fesilicates, plagioclase and K-feldspars dissolution is too low to explain uraninite dissolution and the transport of uranium aqueous species (water in equilibrium with these minerals is found in the stability field of uraninite). 6. Three-dimensional topography-driven flow Due to the deficiencies of the 2D models, we applied a three-dimensional (3D) flow model that is more suited to represent flow focussing. The model includes a set of 9512 nodes and 8176 quadrilateral elements, representing the major tectonosedimentary structures that may have influenced regional flow. The following features are included (Fig. 7): 1. A granitic block with low permeabilities located at the border of the basin. The tensor of hydraulic
conductivities is assumed to be anisotropic with its main component being vertical. 2. The Le´you-Mounana fault as a regional tectonic system that controls sedimentation in the margin between the Lastourville basin and the Franceville basin. 3. Three different units within formation FA characterized by different hydrodynamic properties and their position with respect to the Le´you-Mounana fault. One of these units represents the basal high permeable conglomerates of unit FA. 4. A transversal topographic gradient (WSW–ENE) affecting the plutonic massif and superficial sediments, and a longitudinal regional topographic gradient (NNW–SSE), which is the principal source for topography-driven flow. In order to maintain the oxidant redox state in the fluids before reaching the location of the Oklo ore body, recharge must have taken place through the formation FA sandstones cropping out in the Lastourville basin. The hydraulic connection between formation FA sandstones in the Lastourville basin and higher conglomeratic units of formation FA in the Franceville basin is provided through the Le´youMounana fault. The resulting flow system is mainly parallel to the longitudinal axis of the basin with preferential pathways at the base of formation FA. Flow velocities are
170
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171
Fig. 7. Geometrical schema of the conceptual 3D topography-driven flow model: (a) the basin as a whole, and (b) enlargement of the lower series.
Fig. 8. Flow vectors in the marginal FA conglomerates (in the Oklo area).
J. Salas et al. / Journal of Geochemical Exploration 69–70 (2000) 165–171 ⫺8
171
in the order of 3–8 × 10 m/s with maximum values in the marginal conglomerates. Flow in the plutonic massif is in the order of 10 ⫺10 –10 ⫺12 m/s, several orders of magnitude below. Regional flow is mainly descending and parallel to the base of formation FA. Due to the geometry of formation FA and the assumed characteristics of the Le´you-Mounana fault, the flow system turns on the footwall block into the E–W direction, and finally turns into a vertically ascending flow in the Oklo area (Fig. 8), where hydrocarbons were stored. In this way, the flow system obtained explains the interaction of the oxidizing water with the basal FA sediments and with the reducing fluids in the Oklo area. The 3D model is able to explain the location of uranium ore at the western margin of the Franceville basin as a result of the tectonic structures of the basin and the presence of highly permeable units of formation FA at the basin margin.
flow required paleotopographic gradients, which are poorly supported by geologic data and do not fit well with the geochemical system. The proposed 3D model is capable of reproducing flow patterns that are in accordance with our data. Furthermore, it permits us to explain oxidising solutions with uranium aqueous species in the upper part of the marginal conglomerates of formation FA, where the main flow component is vertical.
7. Conclusions
Gauthier-Lafaye, F., 1986. Les gisements d’uranium du Gabon et les re´acteurs d’Oklo. Mode`le me´talloge´nique de gıˆtes a fortes teneurs du Prote´rozoı¨que infe´rieur: Me´moires Sc. Ge´ologique, 78, Strasbourg, pp. 3–192. Gauthier-Lafaye, F., Weber, F., 1989. The Francevillian (Lower Proterozoic) uranium ore deposits of Gabon. Economic Geology 84, 2267–2285. Olivella, S., 1995. CODE_BRIGHT Manual (not published). Dpt. de Ing. del Terreno—UPC, Barcelona. Salas, J., Ayora, C. Modeling flow and reactive transport at basin scale: 1st joint EC-CEA workshop of the Oklo II working group, Sitges, June 1997.
It has been shown that 2D models are insufficient to explain the generation and location of uranium ores at the Oklo site in the Franceville basin. Thermal convection is not capable of providing sufficient fluid flow and solute for the precipitation of ores. Furthermore, the locations of ore precipitation derived from the thermal convection model do not correspond with the actual location at Oklo. Topography-driven
Acknowledgements This work forms part of the Oklo Phase II project, funded by the EC contract FI4W-CT96-0020, and by ENRESA.
References