Abstract

The lolodorf syenite axis is known for its radiometric indices. In the Ngombas area the migration and retention of uranium in the regolith developed on syenite is studied using a combination of mineralogy and geochemistry in a bid to understand the processes involved in the dissolution and redistribution of uranium in the secondary environment. A trench dug in the area shows three horizons from the bottom to the top. They include the saprolite, B and Ah horizons. Petrographic and XRD investigations of the syenites reveals minerals such as plagioclase, potassic feldspar, amphibole, pyroxene, biotite, quartz, hematite, zircon coupled with uraninite, U-monazite. The regolith developed on the syenite shows relics of plagioclase, amphibole, quartz, hematite, goethite, chlorite, vermiculite, kaolinite. This is associated with U-bearing minerals such as uranothorite, U-monazite, U-zircon, U-florencite and U-rhadophane. The chemical alteration index (CIA), and gain and loss diagrams indicate that the horizons are more weathered from the top to the bottom. The presence of uranium bearing phases such as uranothorite indicates that U-minerals were dissolved, migrated and sorbed on thorite. The occurrences of U-florencite and U-rhabdophane in the weathering blanket indicate that uranyl is stabilized by phosphate minerals. Under oxidizing conditions the stability of hexavalent uranium is favored by the presence of clay minerals and Fe/Mn-oxyhydroxides. Thus, the migration of uranium in Ngombas is sequestrated by clay blended on Fe-oxides through the process of sorption. The U-bearing phases in the regolith that survived weathering include monazite and zircon. The presence of accessories minerals (U-zircon, U-monazite), sorption of uranium by phosphates, by Fe/Mn-oxyhydroxides, and clays minerals play important roles to reduce the U migration in environmental impact of Ngombas region.

1. Introduction

The weathering blanket in the southern region of Cameroon proceeds from the weathering of many rocks types and, characterizes a humid tropical climate zone ^{1, 2, 3, 4, 5}. Generally, weathered products also called laterite, includes (from bottom to top): coarse saprolite, fine saprolite, nodular and loose clay horizon ^{6, 7}. These components of weathered materials are precursory of soil formation ⁶. Increasing weathering breaks down the parent rock which leads to the formation of secondary minerals ⁸ and enables the mobility of chemical elements in the weathering blanket ^{9, 10}. According to ^{8, 10, 11, 12, 13, 14} during weathering, the primary minerals are continually transformed to clay minerals (kaolinite, smectite, vermiculite etc..) and/or Fe-oxyhydroxydes, accompanied by the migration and redistribution of elements. The behavior of chemical element during weathering is controlled by parameters suc as pH, redox conditions, sorption (adsorption/desorption), compatibility/incompatibility and the presence of organic matter ^{13, 14, 15}.

The migration of uranium depends on the redox condition in the parent rock. It has been shown that the climate transforms the primary uranium (U⁺⁴) mineral (uraninite, coffinite, brannerite) in rock to uranyl ions UO₂ (U⁺⁶) in the soils or waste rocks ^{16, 17, 18, 19, 20}. The mobility of uranyl ions (U⁶⁺) in the weathered blanket is inhibited or naturally attenuated by the formation of complexes controlled by sorption process, that influences speciation of uranium in oxidizing conditions ^{21, 22}. According to ²³, the uranyl ions do not migrate in acidic conditions but when stabilized in soil in weak acidic to near neutral pH ranges ²⁰. Secondary U-mineral phases include autunite, schoepite, jarosite. Despite the occurrence of several secondary U-mineral phases, phosphates are the main stable form of uranium in weathering environment ^{20, 22}. In the soil, clay minerals (kaolinite, montmollorinite, illite, clinochlore) and iron oxyhydroxides (hematite, goethite, ferrihydrite) sequestrate uranyl ion (U (VI)) by sorption and/or co-precipitation ^{17, 18, 19, 20, 24, 25, 26}.

Studies of uranium migration in soils of humid equatorial area are rare. Nevertheless, the weathering of the rocks ^{1, 3, 8, 27} and the geological context of uranium mineralization in tropical regions [28-38] have been discussed in detail.

In Cameroon, the research for uranium began around the 1970’s ³⁹. Results revealed radiometrics anomalies in granites and nephelinitic syenites. The occurrence of uranium in granite is reported in Ekomedion ^{36, 37}, Lolodorf and Kitongo ³⁵. Radiometric anomalies have been identified along the Lokoundje River Basin which dissects through radioactive syenite sources ^{40, 41, 42}. Recent research reveals that alpha emitting radionucleides in Melondo and Ngombas attribute radiometric anomalies to the presence of Th, U, Po, Ra in soils and fern ⁴².

In the Southern Cameroon, studies on uraniferous syenite have focused on the description of the weathering profile, the behavior of Ce, REE and Th ^{43, 44}, and the genesis of soils ⁸. In this study we evaluate the petrology, mineralogy and geochemistry of uraniferous syenite as well as the weathering blanket developed on them in a bid to understand the processes involved in the migration of U from the primary rock into the secondary environment in Ngombas Southern Cameroon.

2. Geological Setting

The Nyong complex of Pan-African age in the southern part of Cameroon belongs to the West Central African Belt (WCAB). It was set in place by the remobilization of the Western part of the Congo craton during the Central Eburnean orogeny (2400-1800Ma), with slight shear of juvenile material from the collision between the Congo and San Francisco craton ^{45, 46, 47, 48, 49}. Two tectonics phases have been recorded in the Ntem complex. They include the Eburnean and Pan-African reactivation (Figure 1A) ^{40, 47, 48, 49}. The Nyong complex is characterized by amphibolite, quartzite, metagranitoide, TTG, anorthosite, metagabbro, charnokite, gneiss bearing magnetite, alkaline metasyenites and BIFs are the main petrographic features ^{12, 40, 49, 50}, with soils that are generally to the ferralitic and hydromorphic soil ⁸.

Ngombas is located in the Southern part of Cameroon (3°28 N-10°90 E and 3°21N -10°58 E (Figure 1B). The climate is subequatorial with four seasons marked by two rainy seasons and two dry seasons. The average annual rainfall and temperatures are respectively 1500 mm to 2000 mm and 24°C ⁵¹. The dense rainforest vegetation is affected by anthropic activities ⁵². The morphology is characterized by variable relief reaching 500 to 700 m. The relief is dominated by peneplanes, hills dissected by U and V shape valleys.

3. Material and Methods

During the search of uranium indices in the southern part of Cameroon, a uranium anomaly was identified along the Lolodorf syenitic axis http://www.megauranium.com /main/?kitongololodorf. In order to evaluate the behavior of U in the regolith developed on the bedrock a trench was dug to the bedrock to expose systematically the weathering blanket. The horizons identified were characterized base on their color, texture, structure and mineralogy. Nine samples (rock and soil) were collected from the trenches using the channel chip sampling method. Samples collected were bagged and transported to the University of Douala for further separation and preparation for geochemical analysis.

A section of each sample was used to prepare polished thin sections (rock, saprolite and impregnated horizon B) at the Institut de Recherche pour le Développement (IRD), Bondy Research Center France. The thin sections were observed under the Nikon Eclipse LV100 optical microscope for their petrographic properties. They were then mounted on a Zeiss ULTRA55 microscope in order to determine the mineral phases present.

SEM images using backscattered electron mode (BSE) were recorded at 5, 20 and 25 keV on a Zeiss Supra VP in the ECCE TERRA, at Observatoire des Sciences de I’Univers, Sorbonne University-INSU. The chemical compositions of the U-bearing mineral phases were determined on a CAMECASX-FIVE equipped with five WDS and one EDS detector at CAMPARIS ECCE TERRA, Sorbonne University. The spatial resolution was set at 1 µm². WDS microprobe characterization was achieved for oxides of Al, Si, P, K, Ca, Ti, V, Mn, Fe, Y, Ce, Pb, Th, U, Na and Mg.

Major mineral phases were analyzed by powder XRD. Prior to that the samples were crushed in a marble crusher and pulverized using an agate motar and pestle. The 30 µm size fraction obtained was mounted with a random orientation in aluminium rotating sample holder and analyzed with an X'pert Pro Panalytical diffractometer using an X'Celerator detector and Cu Kα (1.54 Å) or Co Kα (1.79 Å) monochromatic sources at 40 kV and 40 mA. Instrumental conditions were as follows 40 kV, 40 mA, goniometerscan from 3^o to 70^o 2θ with count time of 10800s every 0.016^o. In order to identify the clay mineral fraction, 10 g of bulk samples were mixed with deionized water and ultrasound for 10 minutes in order to separate agglomerated minerals. After 1800s, the float was deposited on a glass-disk holder and air dried to obtain the fag fraction. The oriented samples were glycolated and heated (500°C for 3h) and then scanned from 3° to 15° 2θ with a counting time of 1800s every 0.0167°. An interactive software ESPRIT Bruker was used to identify the main mineral phases present. Identification was based on multiple peak matches using the mineral data base provided by the software.

The chemical compositions of the samples were determined at the ALS Global Chemex Laboratory in Petroria, South Africa. Loss on ignition was achieved by heating the powder samples at 105°C under nitrogen to eliminate water, and at 1000°C under oxygen to extract volatile components. Major and trace elements were analyzed using a combination of inductively coupled plasma-atomic emissions (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) respectively. Prior to analysis, samples were digested in HF, HNO3, and HCLO4 acids. The samples were analyzed at a temperature of 200°C within pressure-tight Teflon cups, and then the dissolved solution was measured on a Perkin Elmer Elan 9000 instrument. Standards and duplicates were analyzed for quality control. The detection limits varies from 0.01 to 0.05 for the major elements and from 0.1 to 0.5 for the trace elements. The geochemical trends during weathering were determined as a function of the degree of chemical weathering using the chemical index of alteration ⁵³. The element dispersion between the basement rock and the regolith were then computed using the mass balance method of with TiO₂ as the least mobile element of choice.

4. Results

Syenite is coarse-grained and gray to greenish in color (Figure 2). It is heterogranular and is composed predominantly of plagioclase, potassic feldspar, amphibole, biotite, quartz, hematite, zircon, apatite, epidote and oxides (Figure 3 and Figure 4). Syenite bears uranium mineral such as uraninite, U-monazite and U-zircon (Figure 5a and Figure 5b). Clay minerals identified include kaolinite and chlorite (Figure 4a). Plagioclase is common in the syenites. It is subangular and in some cases occurs as inclusions in zircon (Figure 3a). Biotite is elongated in shape (Figure 3a). Amphibole and quartz are euhedral (Figure 3a and Figure 3b) and occurs as disseminations in the groundmass. Zircon occurs as inclusion in uraninite. U bearing phases in the syenite occurs in the form of uraninite, U-monazite and U-zircon. U-monazite and U-zircon are associated with apatite and plagioclase respectively (Figure 5a and Figure 5b).

The weathering materials that developed on the syenite reveal three horizons from the bottom to the top: saprolite, B horizon and the Ah horizon. The saprolite is red in color and reveals the structure of the bedrock (Figure 2). The saprolite shows desiccation with micro-cracks sometimes filled with clay and fine gravel. It has a thickness of 25 cm. It reveals relics of plagioclase, potassic feldspar quartz and clay minerals (Figure 3c and Figure 5b). Relics of amphibole are associated with oxides. Uranium-bearing phases identified here include uranothorite, U-florencite and U-monazite (Figure 5c, Figure 5d and Figure 5e). While uranothorite is associated with chlorite (Figure 5c). U-florencite occurs as inclusion in apatite which one is intersected by U-monazite (Figure 5e).

The B horizon preserves only relics of the parent rock and some roots. It is clayey and shows a clotty structure. It is yellowish in color and has a thickness from 55.5 cm and slight porose (Figure 2). Relics of plagioclase and quartz occur as inclusions in clay minerals (Figure 3e). Clay minerals identified here include vermiculites, chlorite and kaolinite (Figure 4c). Uranium phases identified are uranothorite, U-rabdophane, U-zircon and uranium in kaolinite (Figure 5f).

The Ah horizon is 15 cm thick. It is clayey and varies in color from gray to brownish yellow (Figure 2). It is porous contains humus. The weathering materials show relics of amphibole, plagioclase, potassic feldspar quartz and clay minerals (Figure 3f) such as vermiculites, chlorite and kaolinite (Figure 4d). The iron oxides minerals identified here include hematite and goethite blended in clay (Figure 4a).

Electron microprobe analysis on U-bearing phases (Table 1) such as uraninite, U-zircon, U-monazite uranothorite, U-florencite, U-rabdophane and U-kaolinite in syenite and the weathering material shows a variation in the content of UO₂. In the syenite, uraninite is the main uranium bearing phase with a concentration of 80.82 wt% UO₂. The concentration of UO₂ in U-zircon varies from 0.11 to 0.74 wt%. U phases identified in the saprolite show UO₂ content that varies between 0.28 and 1.94 wt% in U-monazite, 0.28 wt% in U-florencite and 26.37 wt% in the uranothorite. The B horizon reveals 24.37 wt% UO₂ in uranothorite, 0.4 wt% in zircon, 0.21 wt% in rabdophane and from 0.14 to 0.16 wt% in kaolinite (Table 1).

The major, trace and REE composition of the syenite and the weathering materials are given in Table 2. Syenite is enriched in SiO₂ (average 60.7 wt%). It shows moderate alkali contents (K₂O averages 7.34 wt%, Na₂O averages 4.55 wt% with an average of 2.55 wt% for CaO). The CIA and LOI values in syenite are 54.57 and 0.48 respectively. Syenite reveals moderate to high concentrations of fluid-immobile elements such as Al (averages 17 wt% Al₂O₃), Zr (1385.5 ppm Zr), Th (203.5 ppm Th), Y (55.8 ppm Y) and Hf (24.9 ppm Hf). U content reaches a high of 50.6 ppm, coupled with elevated concentration of Ba, Sr, Nb, Rb and Pb (Table 2).

High content of SiO₂ (average 59.26 wt%) and moderate concentrations of alkali elements (K₂O average 9.22 wt% and Na₂O average 2.69 wt%) are observed in saprolite. The CIA and LOI values are 60.31 and 2.76 respectively. The concentrations of fluid-immobile elements such as Al (average 19.12 wt% Al₂O₃), Zr (average 1256.2 ppm Zr), Th (average 141.2 ppm Th), Y (average 19.44 ppm Y) and Hf (average 21.16 ppm Hf) are moderate to high in saprolite. The concentration of U in the saprolite horizon reaches a maximum of 66.9 ppm. This is coupled with elevated contents of Ba, Sr, Nb, Rb and Pb (Table 2).

The B horizon shows high contents of SiO₂ (average 56.2 wt%) and moderate contents of the alkalis (K₂O with an average 7.78 %). CIA value shows an average of 71.26 and a LOI which averages 8.67. It is characterized by moderate to high concentrations of fluid-immobile elements such as Al (average 20.4 wt% Al₂O₃), Zr (average 1425 ppm Zr), Th (average 53.7 ppm Th), Y (average 10.55 ppm Y) and Hf (average 25.55 ppm Hf). The concentration of U averages 11.53 ppm. This is coupled with elevated contents of Ba, Sr, Nb, Rb, Zn, Cr and Pb (Table 2).

The Ah horizon shows high SiO₂ content (average 57.6 wt%), moderate alkali concentration (K₂O average 8.3 wt% and Na₂O average 0.4 wt%). CIA and LOI are respectively 64.56 and 9.52. Fluid-immobile elements show high concentration examples Al (18 wt% Al₂O₃), Zr (1760 ppm Zr), Th (59.7 ppm Th), Y (10 ppm Y) and Hf (30.7 ppm Hf). The concentration of U in this horizon reaches a maximum of 12.8 ppm, coupled elevated concentrations of Ba, Sr, Nb, Rb, Zn, Cr and Pb (Table 2).

Gain and loss diagrams were constructed (Figure 6) to depict the element mobility from syenite to the weathering blanket. The weathering materials show depletions in SiO₂, Al₂O₃, MnO, MgO, Na₂O, CaO, K₂O and P₂O₅ excepted of Fe₂O₃ which shows an enrichment in the saprolite horizon NG4.

Several elements are depleted from the sprolite, B and Ah horizons, with the exception of V, Zr, Co, Sc, U and Th which shows enrichments in samples NG3 and NG4 from the saprolite horizon respectively (Figure 6a and Figure 6a-1). In the B horizon, major elements are lost while trace elements such as Cr and Sc are gained (Figure 6b and Figure 6b-1). The Ah horizon shows depletion in the major element, this is coupled with and enrichment in Cr and (Figure 6c and Figure 6c-1).

5. Discussion

The Weathering profile developed on the syenite reveal three horizons from bottom to top: they include the saprolite, B and Ah horizons. Relics of primary minerals identified are plagioclase, amphibole and quartz (Figure 3e and Figure 3f).

The increase of loss element (U, Th, Fe₂O₃, MgO, Na₂O, CaO) observed on the figure of gain and loss justify the degree of weathering of blanket. The B and Ah horizons show the absence of minerals such as amphibole, biotite and pyroxene (Figure 3f). The disappearance of texture from the saprolite to the Ah horizon indicates the increase of weathering rate in the profile. The disappearance of texture from the saprolite to the Ah horizon indicates an increase in the rate of weathering in the weathering profiles. Especially in horizons B and Ah, the relics of primary minerals are surrounded in clayey matrix.

The clay minerals identified in these horizons include chlorite, vermiculite and kaolinite (Figure 5c and Figure 5d).

The abundance of kaolinite in horizon B and Ah shows that the monosialitization processes predominate bisialitization according to ²⁷. The abundance of goethite in the Ah horizon comparatively to saprolite and B horizon attest to increase in meteoritic alteration from the bottom to the top (Figure 5a). The increase of goethite and the clays minerals from saprolite to horizon B attest an increasing the degree of weathering and the effect of meteoric alteration.

Uraninite is the main uranium phase identified in syenite. EMPA analyses of uraninite indicate the presence of thorium (Table 1). This is similar to uraninite reported in granites albitites from the uraniferous province in Ukraine ⁵⁴. Impurities in uraninite suggest the coexistence between uranium and thorium in the melt source. The presence of U, Zr and Y in thorite suggests a solid solution which is the case with the thorite from the Ngombas area ⁵⁵ showed that the presence of U, Zr and Y in thorite suggests the presence of solid solution such as the case with the thorite from the Ngombas area. Uraninite occurs as inclusions in zircon. Inclusion of U-oxide in zircon indicates a magmatic process. Studies of ^{25, 26, 56} reported similar cases of uraninite in primary minerals (quartz, k-feldspar) in granite. This magmatic origin is also supported by the presence of accessory minerals in the syenite such as U-monazite and U-xenotime.

We have shown that the regolith indicates the absences of uraninite. The formation of new U mineral phases in the regolith depends on the oxidizing condition, secondary minerals and phosphates in the weathering blanket. Oxidizing condition induce high dissolution of uraninite and their migration as uranyl (UO₂⁺) ⁵⁷. In the weathering blanket, uranyl is sorbed and/or sequestrated by various mineral phases ²⁰. The saprolite horizon shows uranothorite bearer on chlorite. These minerals suggest the sorption of uranyl on thorite mineral which one is considered as refractory mineral. The sorption process of uranyl reduces the mobility of uranium ^{58, 59, 60}. U-monazite is the only primary mineral that bears U in the regolith. Their resistant to weathering ⁶¹ prevents further migration of uranium. Due to mechanical and chemical weathering, these accessories minerals are generally associated with clay minerals of the weathered products and contribute to the stabilization of uranium in the regolith. Phosphates enhance the adsorption of U in the regolith and contribute to the formation of new uranium phases ⁶². Thus, the U-florencite (CeAl₃(PO₄)₂(OH)₆) aids in the reduction of U mobility and facilitate the retention of uranium in the regolith in Ngombas.

In the B horizon, the presences of uranothorite attest to the resistance of this uranium mineral and play an important role in the mobility and retention of uranium in Ngombas profile. In this horizon, zircon identified bears uranium. Zircon is a resistant mineral during weathering process ⁶¹ Just like the monazite in saprolite horizon, zircon contributes to the stabilization of uranium in this horizon ²⁰. Phosphate minerals such as apatite, monazite and other primary phosphates participate to the sequestration process of uranium ^{20, 21, 22}. EMPA analyses reveals as high as 0.21 wt% of U in rabdophane (Ce, La (PO₄) (H₂O)) (Table 2). U sorption in clay minerals could be tied to high cationic capacity and organized crystalline pattern ¹⁹. Fe-oxides and clay minerals play an important role in actinides migration such as radionucleides, trapping uranyl as inner- and/or outer-sphere complexes at their surface ^{20, 25, 64, 65}. This sorption is more evidence in clay mineral blends with high Fe-oxides contents ⁶⁴. The weathering blanket reveals as high as 0.15wt% U in kaolinite. The concentration of U becomes higher as kaolinite blends with Fe-oxides (0.15 wt%). Thus the retention of U in this horizon is attributed to the presence of clay minerals.

In horizon Ah, the absence of new uranium phases suggests that oxidizing condition induce the solubility of previous U phases ^{22, 23}. Nevertheless, the presence of organic matter in this horizon suggests the U was sorbed as nanocrystals of uraninite in organic matter ⁶⁶.

U migration in waste rock and/or in natural soils depends on oxidizing condition, pH and U-speciation ^{20, 64, 66}. Generally, U is mobilized as uranyl and/or uranyl phosphate in groundwater or associated in particles in the environment ^{26, 69}. The source of U in Ngombas profile is attributed to dissolution of uraninite inclusion in zircon. The other U-bearing mineral present is monazite, which is considered as accessory mineral and resists to meteoritic weathering ⁶¹. The weak weathering rate of zircon and monazite could explain the feeble content of uranium in the horizon. Nevertheless it could suggest another source not identified in this study.

The presence in soil profile of uranothorite shows that uranyl mobilized is sorbed on thorium mineral and contribute to reduce the impact of uranium in the environment. Reference ²⁰ reported that phosphate minerals such as apatite, monazite and other primary phosphates play the important role in U sequestration. U-florencite and U-rhabdophane present in the Ngombas profile attest that the phosphates minerals sequestrated U-dissolved and limiting the mobility of (UO₂²⁺) in the subsurface environment. References ^{18, 19, 70} reported that clay minerals and Fe-oxides contribute to reduce the impact of uranium in the environment by their sorption capacity. As the previous research, the presence of uranium content in kaolinite and the increasing of this content in Fe-oxides blend kaolinite show the impact of clays and oxides in the migration of uranium in the Ngombas environment. Also, the presence of phosphate and vegetation could promote the formation of (nanocrystal) uranyl phosphates, thereby limiting the migration of uranyl in Ngombas surface environments ⁵⁹.

6. Conclusion

The soil profile of Ngombas is developed on the syenite. Three horizons are distinguished from the bottom to the top: saprolite, B and Ah horizons. The syenite is composed of plagioclase, potassic feldspar, amphibole, pyroxene, biotite, quartz, hematite and zircon. The horizons show relics of plagioclase, amphibole, quartz, hematite and goethite. Clay minerals are more abundant from the top to the bottom of profile and include: chlorite, vermiculite and kaolinite. The intensity of weathering increase from the bedrock to the Ah horizon. CIA values vary from 56.9 to 71.26 % respectively from the syenite to Ah horizon. Also, gain and loss of chemical elements show the decreasing of major elements from saprolite to Ah horizon.

EMPA analyses show the uraninite in zircon inclusion in the syenite. U-monazite and U-zircon are the accessories minerals bearing uranium in the rock. Uraninite is not identified in saprolite, uranothorite is a new phase presents in the horizons. It is associated to U-zircon and U-monazite with the new phospahte mineral U-florencite and U-rhabdophane. That minerals and Fe-oxides blended kaolinite in soil profile contribute to limit the mobility of uranium in the Ngombas environment.

Acknowledgements

The authors wish to acknowledge funding from IMPMC (Institut de Minéralogie et de Physique des Matériaux et de Cosmochimie) that covered the research stay of JSM at the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie in France. They also thank the engineers of the OCEAN Department of IRD, Bondy France, T. Pilorge, S. Caquineau, for their technical assistance and advice.

References