Maurice Pagel

Cerium anomalies in lateritic profiles

The REE geochemistry and mineralogy have been studied in four lateritic profiles, one derived from a syenite (Akongo, SW Cameroon), the others being developed on a gneissic basement and located along a soil toposequence (Goyoum, E Cameroon). There is a fractionation between LREE and HREE in the lateritic samples during weathering, the weathered residual products being enriched in LREE (from La to Eu) and depleted in HREE (from Gd to Lu); sampled waters are enriched in HREE in relation to the syenite host-rock. A positive Ce-anomaly has been found systematically at the top of the saprolite, beneath a zone of iron oxide accumulation in the laterite. Up to 2000 ppm Ce may be present. In the Akongo profile, cerianite, CeO2, is present as very fine coatings in non-ferruginous clayey domains. Primary REE-bearing accessory minerals are weathered at the bottom of the profile. Specifically, allanite and apatite are transformed into florencite and rhabdophane but these phases have no Ce-anomaly. All the data are interpreted as the result of the following processes: n1. n(1) REE leaching in a reducing environment. n n2. n(2) oxidation of Ce3+ to Ce4+ in an oxidizing environment. n n3. n(3) deposition of cerium as cerianite whereas the other REE remain in solution.

Radiolysis evidenced by H2-O2 and H2-bearing fluid inclusions in three uranium deposits

H2 with or without O2 has been identified by Raman spectroscopy in fluid inclusions in quartz from three Precambrian uranium deposits: Oklo (Gabon), Rabbit Lake and Cluff Lake D (Saskatchewan, Canada). In the Oklo uranium deposit, heterogeneous trapping of fluids in the system H2O-H2-CH4-salts is spatially associated with the natural nuclear reaction zones. At Rabbit Lake and Cluff Lake D, fluid inclusions show heterogeneous trapping of an aqueous liquid phase and a vapour phase with variable X(O2)X(H2) ratios always greater than one. n nH2 and O2 formation is discussed in terms of chemical reactions, chemical reactions indirectly induced by nuclear reactions and radiolytic processes. Numerical calculations of water radiolysis were carried out for two models: 1) water at the contact with an uranium oxide and 2) water inside a fluid inclusion containing either dissolved uranium or a small UO2 crystal. The concentrations of O2 and H2 inside fluid inclusions from Rabbit Lake and Cluff Lake D deposits are consistent with the first model. n nIn the Oklo deposit, the H2-bearing fluid inclusions found in the quartz and the anomalous high OC atomic ratio of organic matter from nuclear reactor zones could result from radiolysis of water and organic matter due to the fission products of the chain nuclear reactions, but α radiolysis is not definitely ruled out.

The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16

In an underground nuclear waste repository, the chemical behavior of some stored fission products and actinides depends on the redox conditions during their long-term evolution. In this respect, radiolysis is an important phenomenon which can significantly modify the local redox conditions. The Oklo natural fission zones are good examples where the effect of radiolysis can be deduced from a mineralogical and geochemical study. Zones 10 and 16 were studied because they are located at depth of 270 m in an area devoid of any recent water circulation and not subject to the effect of the lateritic alteration occurring elsewhere in this area. n nIn zone 10, there is a marked evolution of the Uue5f8Pbue5f8Feue5f8S mineralogy from the center to the periphery of the reactor zone. In the center, uraninite shows silicification and coffinitisation with the formation of galena and native lead; the PbO content of uraninite can be as much as 20 wt%. In the periphery of the reactor zone, some radiogenic lead is present as minium (Pb304) and in Pb-bearing calcite. In the surrounding sandstones, hematite is widespread. n nIn zone 16, the mineral paragenesis is generally comparable with that of zone 10 but with some differences. Galena is the only Pb-bearing mineral associated with uraninite crystals. The PbO content of uraninite is always <7 wt%. In the periphery of the alteration zone, barite partly replaces quartz. In the reactor zone, hematite is sometimes replaced by pyrite. n nIn an area where the fission zone 10 is in contact with sandstones devoid of organic matter, H2Oue5f8H2O2 and H20ue5f8H2 ± CH4 inclusions were observed in healed microcracks in the detrital quartz grains. Based on microthermometric measurements, the salinity of the aqueous solution ranges from 0.2 to 18 wt% eq. NaCl. Raman analysis of the gas phase indicates that the hydrogen to oxygen ratio differs from an inclusion to the other. n nThe presence of H2− and O2-bearing fluid inclusions confirms the existence of water radiolysis in the reactor zone. In zones rich in organic matter, oxygen was preferentially consumed in reactions with organic matter and its radiolytic products. In the absence of organic matter and its radiolytic products, oxidizing conditions prevailed and hematite and minium have formed. Consequently, presence of highly oxidizing conditions has important consequences on uranium and fission products mobility. Water radiolysis is also evidenced during the subsequent 1.97 By history of the reactor. n nDuring the fission reactions, redox conditions deduced from Pb-bearing minerals indicate that the sulfur fugacity was very low in zone 10 as shown by the formation of minium and higher in zone 16 with the formation of barite and pyrite. In this paper we discuss the geochemical consequences of such contrasting redox zones and the resulting redox heterogeneity created by water radiolysis.

Radiation-Damage Rims in Quartz from Uranium-Bearing Sandstones

ABSTRACT Using cathodoluminescence (CL) mode of a scanning electron microscope (SEM), radiation damage in quartz is made visible in uranium-vanadium-bearing sandstones located in the Salt Wash Member of the Morrison Formation (Jurassic) from the Colorado Plateau (U.S.A.). Detrital quartz grains surrounded by uranium minerals display radiation-damage rims. Three concentric rims with thicknesses of 37 µm, 24 µm and 16 µm have been observed. The rims are similar to halos surrounding uranium- and thorium-rich accessory minerals observed by Owen (1988). They are due to alpha particles, which create damage when penetrating a medium. The width of the rims agrees well with the Bragg-Kleeman rule and corresponds to the most energetic alpha particles in the 238U decay series Sandstones entirely cemented by vanadium clays also exhibit radioactive rims, which demonstrates that uranium has been removed during diagenesis. These results have important implications for the uranium ore genesis in the Salt Wash sandstones, because they imply an ancient episode of uranium leaching prior to recent weathering. Therefore, cathodoluminescence in sandstones may be useful in recognizing ancient zones or episodes of accumulation and leaching of alpha-emitting nuclides from the uranium series.

Thermal History Constraints from Studies of Organic Matter, Clay Minerals, Fluid inclusions, and Apatite Fission Tracks at the Ardeche Paleo-margin (BA1 Drill Hole, GPF Program), France

ABSTRACT The thermal history of the Ardeche paleo-margin (southeastern France) has been reconstructed using various analytical data from minerals and organic matter and conventional interpretation. These methods comprise Tmax and vitrinite reflectance measurements on organic matter, determination of the smectite content in mixed-layer illite/smectite, fluid-inclusion microthermometry in quartz, barite, dolomite, and anhydrite, and fission-track analyses in apatite. The Balazuc (BA1) drill hole, 1730 m deep, intersects mainly Jurassic carbonate rocks, Triassic sandstones, and sulfate-rich claystones and ends in Carboniferous rocks. The present bottom-hole temperature is 70°C. Other drill holes, less than 6 km away, were studied when necessary. One of the main objectives of this paper is to ssess the agreement between the different geothermometric methods, and to check their limitations and convergence when used on carbonate- and sandstone-dominated reservoirs. In the Triassic sandstones, primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one-phase brine inclusions entrapped below 70°C. The homogenization temperature determined on the later barite cement is about 100°C. Aqueous two-phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults. Temperatures as high as 210°C recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the Early Jurassic. The fluid inclusions in dolomite, most of them reequilibrated during burial, indicate a trapping temperature of 130-145°C at 1600 m and a temperature decrease towards the surface. This value is in good agreement with the MPTB (maximum paleotemperature of burial) method on organic matter, which converges to a maximum burial temperature of 130°C at the same depth. The apatite fission-track data agree with these temperatures and indicate that the temperature decreased below 120 ± 10°C during the Eocene. These paleotemperatures, much higher than the present ones, can be explained by the erosion of 1900 m of mainly Cretaceous sediments. This interpretation implies a high average sedimentation rate during the Cretaceous, in agreement with data determined on a regional scale. A time-temperature evolution is proposed for the formations present in the Balazuc drill hole and the eroded sedimentary rocks. The application of the dual reaction model of the transformation of smectite to illite (Velde and Vasseur 1992) shows that the data are in good agreement with the kinetic model. However, the vitrinite reflectance variation trend with depth, predicted from the Burnham and Sweeney model (1989), is different from the data variation. No definitive explanation can be proposed for the failure of aturity assessment through vitrinite reflectance in the BA1 drill hole.

Geochemical and mineralogical behavior of REE, Th and U in the Akongo lateritic profile (SW Cameroon)

Abstract The REE-U-Th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite (Akongo, SW Cameroon). Mass balance calculations have been done (1) in the syenite to determine the most important REE-U-Th bearing minerals and (2) in the soil profile to understand the movement of these elements. Thorium has been used as an immobile element for these calculations. The weathering of accessory minerals rich in REE and Th of the syenite (allanite, apatite, titanite and epidote) has been especially studied in the saprolite. Several REE-Th secondary minerals (florencite, rhabdophane, cerianite and thorianite), important to explain the fractionation of REE in supergene environments, have been found.

Thermo-Chronological Approach to Reservoir Diagenesis in the Offshore Angola Basin: a Fluid Inclusion, {40}Ar-{39}Ar and K-Ar Investigation (1)

Microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in Neocomian sandstones and Albian dolomitic reservoirs from the offshore Angola basin have allowed determination of the age and the physical-chemical conditions for fluid migration in a late diagenetic stage. The {40}Ar-{39}Ar laser probe dating of adularia overgrowths from the Albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early Miocene (25.6 + or - 3.2 Ma). The K-Ar age of authigenic illites from the Neocomian sandstones is slightly younger (19 + or - 1 Ma), and the crystallization of regular illite-smectite mixed layers is related to a previous Eocene to Paleocene diagenetic stage (51-58 Ma). There appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the Miocene. The fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids, assuming that pressures during the Miocene were close to the present reservoir pressures, and by intersecting the isochores of aqueous and oil inclusions, assuming they were trapped at strictly similar pressure and temperature conditions. The fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content (AlIV) of authigenic chlorites: 168-206 degrees C in the Albian reservoirs at depths between 2350 and 3580 m, and 175-237 degrees C in the Neocomian sandstones at depths between 2770 and 4470 m. In both reservoir formations, Miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient.

Organic inclusions in salt. Part I: Solid and liquid organic matter, carbon dioxide and nitrogen species in fluid inclusions from the Bresse basin (France)

Abstract Organic inclusions from the Etrez and Cormoz areas of the Paleogene Bresse salt basin (France) have been studied. Different organic fractions are trapped with brine solutions in fluid inclusions in halite. Isolated inclusions from both areas are brine-bearing, and organic matter is sometimes associated with daughter mineral phases (carbonate, sulfate, clay). Structured and unstructured solid organic matter is observed both in the Etrez area (central basin) and in the Cormoz area (Northern ridge), while liquid organic droplets are only observed in fluid inclusions from the Cormoz area. FTue5f8IR microspectroscopic measurements show a low aliphatic contribution, and the presence of aromatic and oxygen double bonds for the solid organic matter. Low to high aliphatic contents (C5 and C7+, deduced from CH 2 CH 3 ratio measurements), low to high Cue5fbC and Cue5fbO contributions, and the presence of characteristic Cue5f8N functions of amino-acids are detected in liquid droplets by FTue5f8IR spectroscopy. Carbon dioxide, originating from the thermal degradation of solid organic matter, is generally detected. Organic two-phase inclusions (brine + liquid organic matter) in planes are located in clear halite crystals from the Cormoz area. High aliphatic contributions, and variable aromatic and oxygen contents are observed. Ammonium ions (NH4+) are detected in the brine solution, and no carbon dioxide contribution is recorded. Organic matter trapped in the halite of the Bresse basin is immature to early mature. In the deeper part of the basin (Etrez, 800–1000 m), the organic matter in the inclusions is slightly thermally degraded, with little or no generation of liquid organic compounds. This agrees with the data on organic matter previously published from this area. In the Northern ridge (Cormoz, 500–800 m), liquid heteroatomic compounds (C, H, O, N) were produced, migrated through the salt and were trapped without gas phases. This implies a slightly lower temperature in the deeper part (Etrez) of the Bresse basin. Differences in thermal gradients between the Northern ridge, and the deeper part of the Bresse basin, must be invoked to explain differences in organic matter maturation. The analysis of organic inclusions is a contribution to understanding the thermal and biochemical history of the Bresse basin by the in situ observation of organic evolution products which are preserved by entrapment in salt.

A model for238U radioactive daughter loss from sediment-hosted pitchblende deposits and the Late permian-early triassic depositional U-Pb age of the Müllenbach uranium ore (Baden-Württemberg, F.R.G.)

Abstract Abnormally high radiogenic 207 Pb/ 206 Pb ratios in pitchblende from the Mullenbach uranium deposit (Baden-Wurttemberg, Federal Republic of Germany) result in calculated Pb Pb apparent ages which are older than the host Upper Carboniferous sediments ( > 320 Ma), suggesting disequilibrium in the 235 U or 238 U decay series. A theoretical model is proposed to explain this phenomenon based on a correlation between 206 Ph loss relative to 238 U in pitchblende and the apparent ages. The model shows the possibility of calculating the 238 U r.d. (radioisotope daughters) /Pb loss ratio and consequently the true age of mineralization in U-disseminated deposits. Calculated losses of 237 U r.d., chiefly 226 Ra and/or 222 Rn, are proportional to Pb radiogenic losses, in the percent ratio of 0.046. The proportionality is observed by the alignment of most samples in a U Pb concordia plot. The “isochron” gives the age of mineralization [239±6 Ma (2α)] at the upper intercept with the concordia. In the Mullenbach deposit, loss of 222 Rn by emanation is more likely than diffusive loss of 226 Ra because Ra is adsorbed on pitchblende. The high deficit of 238 U r.d. in relation to Pb radiogenic loss (−54%) in the black shales with organic matter associated to the U ore is consistent with this hypothesis. The amount of calculated 222 Rn leakage in the pitchblende (∼ 2.5%) is consistent with an α-recoil model for a 1-μ-diameter pitchblende sphere, the size observed under the scanning electron microscope.

Organic inclusions in salt. Part 2: oil, gas and ammonium in inclusions from the Gabon margin

Abstract Organic inclusions have been found in a salt diapir south of Port-Gentil in the North Subbasin of the Gabon margin. Two main types of organic inclusions are described. Type 1 gas-rich (CH4 and CO2) oil inclusions located in recrystallized halite, associated with a complex mineral daughter paragenesis: anhydrite, quartz, K-feldspar, dolomite, ammonium sylvite and carnallite (1 and 2.5% molar NH4Cl, respectively). The oil is essentially aliphatic with some aromatic content, the composition being characteristic of mature oil. The different methane/C2 + ratios in the gas and oil phases indicate an internal pressure close to 200 bars, which is compatible with hydrostatic pressure at sampling depth. Homogenization temperatures of two-phase hydrocarbon inclusions inside dolomite daughter crystals have been measured (Th = 113°C). Type 2 organic inclusions are located in healed fracture planes in recrystallized halite and contain aliphatic oil; the brine content is high, and no gases have been detected. Different alkane compositions occur in different fracture planes. Type 2 inclusions correspond to injections of mature oil into halite fracture systems at low temperature (≈60°C). Comparisons between fluid inclusions in halite occurring in the Bresse area (France), and in the North Subbasin of the Gabon margin, help one to follow carbon and nitrogen cycles from diagenesis to catagenesis. The organic inclusions hosted in salt rocks reflect the geochemical reactions that have occurred in the salt over time.