Origin and evolution of Ngaye River alluvial sediments, Northern Cameroon: Geochemical constraints

Journal of African Earth Sciences 100 (2014) 164–178

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Origin and evolution of Ngaye River alluvial sediments, Northern Cameroon: Geochemical constraints Paul-Désiré Ndjigui a,⇑, Anicet Beauvais b, Soureiyatou Fadil-Djenabou a, Jean-Paul Ambrosi b a b

Department of Earth Sciences, University of Yaoundé 1, P.O. Box: 812, Yaoundé, Cameroon Aix-Marseille University (AMU), IRD, CNRS, CEREGE UM34, BP 80, 13545 Aix-en-Provence, Cedex 4, France

a r t i c l e

i n f o

Article history: Received 30 May 2013 Received in revised form 1 June 2014 Accepted 8 June 2014 Available online 24 June 2014 Keywords: Alluvial sediments Trace elements Rare-earth elements (REE) Weathering Granitoids Northern Cameroon

a b s t r a c t The origin of Ngaye River alluvial sediments and the evaluation of the weathering degree of their source rocks are assessed using trace and rare-earth element geochemistry in three bulk sediments and their different size fractions (2000–200 lm, 200–50 lm, 50–2 lm and <2 lm). The alluvial sediments consist of two sandy clay layers at the bottom and one sandy heavy clay layer at the top. Quartz and feldspars are the main minerals in the sand fractions while kaolinite and smectite are dominant in the finest sediments. The relatively low Chemical Index of Alteration (CIA) indicates that the sediments and their potential source rocks are moderately weathered. Highest trace element contents are observed in the fine sands, which are the richest in Zr, Th, U, Sc and REE. La, Ce and Nd are the most abundant REE in this fraction. The coarse fractions are characterized by LREE-enrichment relative to HREE. The PAAS-normalized REE patterns exhibit large positive Eu anomalies (Eu/Eu 3.1 to 3.9) in the coarse sand fraction of the sandy clay layers and strong negative Eu anomalies (Eu/Eu 0.35 to 0.70) in the two sand fractions of the sandy heavy clay layer. Our results document the immaturity of the Ngaye River sediments, which derive mainly from the erosion of moderately weathered granitoids of the surrounding reliefs, and in some extent from greenstones and/or basic volcanics. The results also suggest an obvious dependence of trace and rare-earth element fractionation on mineral sorting and weathering in the different grain-sized fractions of the alluvial sediments. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Geochemistry of clastic sedimentary deposits usefully documents the source rocks that fed them (McLennan et al., 1993). Clays of alluvial sediments are either inherited from weathered source rocks or neo-formed by post-deposit weathering or diagenetic processes (Singh and Rajamani, 2001; Njoya et al., 2006; Ngon Ngon et al., 2009, 2012; Singh, 2009; Lawal and Abdullahi, 2010; Nguetnkam et al., 2011). Notwithstanding the primary control of source rocks composition, the grain size and geochemistry of sediments also depend on weathering and diagenetic processes (Sawyer, 1986). Trace element distribution including rare-earth elements (REE) was previously studied in weathering profiles developed upon various rocks under different climate conditions (e.g., Nesbitt, 1979; Braun et al., 1990; Beauvais and Colin, 1993; Boulangé and Colin, 1994; Ndjigui et al., 2008, 2013; Beauvais, 2009; Etame et al., 2009). In Southern Cameroon, REE fractionated in weathering

⇑ Corresponding author. Tel.: +237 9954 3774; fax: +237 2222 6262. E-mail address: [email protected] (P.-D. Ndjigui). http://dx.doi.org/10.1016/j.jafrearsci.2014.06.005 1464-343X/Ó 2014 Elsevier Ltd. All rights reserved.

profiles showed depletions in the upper ferruginous horizons and enrichments in the lower saprolite layers (e.g., Braun et al., 1993). Geochemistry of Kaveri River alluvial sediments in South India (Singh and Rajamani, 2001) revealed that REE concentrations depend on the sedimentary particles size, the finest sediments being richer in REE than the coarse sand fraction and bulk sediment. Skolkovitz (1988) showed that river-borne sediments are strongly depleted in HREE relative to shale. Concentrations of Al, Fe, Ti, Th, Sc, Co, Zr and REE in the finest sediments (50 < Ø < 2 mm) are commonly linked to the host primary minerals including accessory minerals (Taylor and McLennan, 1985; Singh, 2009; Bhuiyan et al., 2011). It was shown that a positive Eu anomaly occurred in the sand fractions, whereas the finest sediments were rather characterized by a negative Eu anomaly. The positive Eu anomaly was related to feldspars, which together with quartz, are the main components of the coarse sand fraction (Singh and Rajamani, 2001; Singh, 2009). Here, we constrain the origin of the alluvial sediments in the Ngaye River valley (Northern Cameroon) by the quantification and distribution of major, trace and rare-earth elements in different grain-sized fractions of these sediments. We also discuss the potential links between the REE behavior and the sorting and weathering degree of the sediments.

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composed of sedimentary and meta-igneous rocks that later underwent medium-to-high grade Pan–African metamorphism (Toteu et al., 2001; Tchameni et al., 2006). The sedimentary formations are constituted from the bottom to top by: (i) conglomerates with rounded pebbles; (ii) fine-grained sand mixed with clay patches; (iii) basanite flow; (iv) marl and probably clays; and (v) coarse conglomerates (e.g., Guiraudie, 1955; Lasserre, 1958; Koch, 1958). The age of the upper part of this series is Middle Cretaceous by correlation with the Lame series in Chad (Le Maréchal and Vincent, 1971). The basement of the Ngaye region is mainly constituted of granitoids (Fig. 1b). The topography of Ngaye region is characterized by high reliefs with a maximum altitude of 1300 m in the NW and SW parts (Fig. 2a and b), which dominate vast alluvial plains (Dirasset et al., 2000) drained by the Vina and Mbéré Rivers (Fig. 1b). Two topographic levels are prominent (800–1000 and 1000–1200 m, Fig. 2a). The landscape of the Ngaye River watershed consists of a succession of hills separated by small and narrow valleys with eroded or gullied bell-shaped bottoms (Figs. 1b and 2a). The soils exposed on the interfluves are 10 m thick, mainly ferruginous, and consist of six horizons from the bottom to top: coarse and fine saprolites, two nodular horizons embedding an iron duricrust, and

2. Geographical and geological setting The Ngaye River watershed is a tributary of the Mbéré River draining the Cretaceous Trough in Northern Cameroon (Figs. 1 and 2a). The Mbéré Cretaceous Trough is deep and located from the northern border of the Adamawa Plateau in Cameroon to the southern prolongation of the Yade Massif in Central African Republic (CAR). The investigated Ngaye alluvial deposits are located at 7°110 4000 N and 14°580 5000 E (Fig. 1a). The climate is semi-arid, with a mean annual rainfall of 970 mm, and a mean annual temperature of 27 °C (Molua, 2006). The vegetation is savanna associated with dense forest galleries in the valleys (Letouzey, 1985). The Adamawa-Yade region is characterized by a NE–SW elongated regional-scale plutonic complex which intrusion into a Paleoproterozoic basement, which is locally covered by Cretaceous deposits (Mbéré and Koum basins) and Cenozoic rocks such as andesitic basalts belonging to the Cameroon Volcanic Line (Guiraudie, 1955) Southwest of Ngaye (Fig. 1b). The AdamawaYade batholith consists of a great variety of more or less deformed rock-types of different ages. The lithologies include granites, diorites, gabbros and syenites (Tchameni et al., 2006). The granites were emplaced into an Eburnean (2.1 Ga) remobilized basement

15°00'E

15°30'E

NIGER 8°00'N

8°00'N

Ndjamena Kano CHAD rde

Bo

NIGERIA

r

Fig. 1b

Abuja

Touboro Ecole Vogdjom

Vina

ne

Logo

Vina

e Ye b

m

Li

7°30'N

ré

bé

Bi

nd

250 km

M

Divide

7°30'N

Yaounde

a

Ugara

Baibokoum

Douala

a

Malabo

CAR

Obogo

CAMEROON

Dompta

MNT SRTM (90m) UTM projection Elevation (m) ré

Ngaye

400 - 600

bé

Bouloupou

M

600 - 800

Ngou

Bertoua

Rivers 15°00'E

N 20 km

b

15°30'E

Precambrian syntectonic granitoids (calco-alcaline granites)

Cretaceous formations (sandstones, marls, limestones)

Cambro-ordovician syn- and post tectonic granitoids (alcaline granites)

Basalts (andesites)

Granitoids with greestones facies (gneiss and migmatites; leptynites, quartzites, amphibolites, pyroxenites)

7°00'N

1200 - 1945

Lim

1000 - 1200

Sokorta Manga

Study site

Bor der

Fig. 2a

7°00'N

800 - 1000

Faults Scarp faults with mylonites

Fig. 1. (a) Location of the study site and (b) combined geological and geomorphological map of this site and Ngaye region (geological map adapted from Le Maréchal, 1976).

P.-D. Ndjigui et al. / Journal of African Earth Sciences 100 (2014) 164–178

NE 1100 1060 Ngaye

Ngaye

980

Ngaye

Ngaye River 940

b a

5 km

N

1020

Elevation, m

SW

Ugara

166

ré

0

1

2

bé

M

Study site

3 4 Length, km

5

Sample location

upper

0.2 MD3

0.6

1.2 1.4 1.6 1.8

Thickness, m

1.0 MD2

lower

Sandy clay

0.4

0.8 middle

Sandy heavy clay

0

2.0 2.2 MD1

2.4

Gravelly bed

2.6 2.8 3.0 3.2

c

3.4

Fig. 2. (a) Geomorphological and drainage map of the Ngaye River watershed, and location of the topographic cross section (b) showing the Ngaye River incision, and the investigated alluvial terrace. (c) Vertical section across the alluvial deposits of the Ngaye River valley with collected samples (white squares) in the different alluvial sedimentary layers. For the legend of (a), see Fig. 1b.

the loose clayey horizon exposed at the ground surface (Brabant and Gavaud, 1985). The loose clayey horizon may sometimes connect with the alluvial deposits in lowlands.

sieving. The size fractions above 50 lm were determined by dry sieving. 3.2. Analytical techniques

3. Sampling and analytical techniques 3.1. Sampling and separation techniques Three bulk sediment samples, one hundred kilograms each, were collected in each layer of a profile in the terrace of the Ngaye River (Fig. 2c). Samples were split up homogeneously and prepared for granulometric, mineralogical and geochemical analyses. Grain size was measured according to the standard pipetting method for the silt- and clay-sized fractions (Singh, 2009). After that, 20 g of the bulk sediment samples were treated with cold 1 M HCl and H2O2 to remove organic matter. Adding sodium hexametaphosphate deflocculated the samples. An aliquot of the bulk sediment below 2 mm grain-sized was separated in four fractions: coarse sands (2000–200 lm); fine sands (200–50 lm); silts (50–2 lm) and clays (<2 lm). Sands were separated using wet

The size fractions and the bulk samples were processed for mineralogical and chemical analyses at the Geoscience Laboratories (Sudbury, Canada). Powders were first prepared in the Department of Earth Sciences (University of Yaoundé 1, Cameroon). Heavy minerals (160–100 lm) were concentrated by density separation using heavy liquid (bromoform) and identified under a binocular microscope. About two hundred grains of heavy minerals were collected for each sample; data are expressed in weight percent. The mineral compositions were determined by X-ray Diffraction (XRD) analysis according to two steps. Bulk and grain size sediment samples were ran using PAN Analytical X’PERT PRO diffractometer in the Geoscience Laboratories at 40 kV and 45 mA. Oriented samples of the clay- and silt-sized fractions were characterized using a Philips instrument (PW1050/81, PW3710) in the Aix-Marseille University (Aix-en-Provence, France). This analytical instrument is equipped with a monochromator using a Co Ka radiation of 1.7854 Å over a range 2.5° to 35° 2h with a step size of 0.05° 2h/min at 40 kV

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and 40 mA. The oriented sub-samples were prepared by mounting the clay- and silt-sized fractions on glass slides. Clay mineralogy was determined by applying different treatments on sub-samples: untreated, ethylene glycol, hydrazine and heated (550–600 °C). The untreated, heated and ethylene glycol sub-samples were X-ray analyzed for 30 min (2.5 s counting time), and the hydrazine sub-samples for 10 min (1 s counting time). Samples for geochemical analysis were crushed using a jawcrusher with steel plates, and pulverized in a ball mill made of 99.8% Al2O3. Powders were first heated at 105 °C in the presence of oxygen to drive off remaining volatile components and oxidized Fe. The loss on ignition (LOI) was determined at 1000 °C on dry (105 °C) samples. X-ray Fluorescence (XRF) was used to determine the major element concentrations after LOI measurement. The sample powders were first ignited and melted with a lithium tetraborate flux and then analyzed with a Rigaku RIX-3000 wavelengthdispersive X-ray Fluorescence spectrometer. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) was used to quantify trace and rare-earth element concentrations. Sample powders for ICP-MS analyses were digested with a mixture of HCl and HClO4 at 120 °C in sealed Teflon containers for one week, and then rinsed out of their containers with dilute HNO3 and dried. The residue was again dissolved in a mixture of three acids (HNO3, HCl and HF) at 100 °C. Sample solutions were analyzed in a Perkin Elmer Elan 9000 ICP-MS instrument.

The geochemical results (major, trace and rare-earth elements) were compared to the average compositions of granitoids such as biotite ± muscovite granitoids (BMG), biotite granitoids (BG) and hornblende-biotite granitoids (HBG) (see Tchameni et al., 2006) which could be the potential source-rocks of Ngaye River alluvial sediments (see Fig. 1b). 4. Results 4.1. Granulometric and mineralogical characterization The Ngaye alluvial deposits consist of three layers (Fig. 2c) which have similar proportions of sands but are differentiated by their quantities of silts and clays (Table 1). The lower layer (MD1) is gray with 1.3–1.6 m thick (Table 1). The middle layer (MD2) is dark gray with yellowish brown patches, and is 0.5– 1.1 m thick. Both layers have a sandy clay texture (Table 1). The upper layer (MD3) is 0.5–0.9 m thick, yellowish brown with sandy heavy clay texture (Fig. 2c), defined by its richness in clays relative to silts (Table 1). The heavy mineral fractions of the samples are mainly composed of epidote, olivine, and opaque minerals, and accessory garnet and zircon (Table 2). These minerals have essentially angular forms that may indicate moderately weathered materials and very close proximity of the source rocks, such as the granitoids defined

Table 1 Particle size distribution (weight in %) in different layers of the Ngaye River sediments.

Upper layer Middle layer Lower layer

Ref. code

Coarse sands

Fine sands

Silts

Clays

Total

Texturea

MD3 MD2 MD1

16.99 14.47 7.20

39.01 43.34 48.70

0.83 8.24 16.50

43.15 33.93 27.30

99.98 99.98 99.70

Sandy heavy clay Sandy clay Sandy clay

Coarse sands: 2000 < U < 200 lm; fine sands: 200 < U < 50 lm; silts: 50 < U < 2 lm; clays: U < 2 lm. a From Ngon Ngon et al., 2009.

Table 2 Mineral distribution of the heavy mineral concentrates in Ngaye River sediments.

Upper layer Middle layer Lower layer

Ref. code

Epidote

Olivine

Opaque minerals

Augite

Zoisite

Diopside

Garnet

Zircon

Tourmaline

Non-identified

MD3 MD2 MD1

12.00 13.00 32.50

29.00 40.00 29.50

45.00 20.00 9.00

0.50 0.50 4.50

– 0.50 5.00

1.00 – 2.50

4.00 5.00 2.00

3.00 1.00 1.00

1.00 0.50 0.50

4.50 19.50 12.00

Table 3 Mineralogical composition of Ngaye River sediments. Lower sandy clay layer – MD1

Middle sandy clay layer – MD2

Upper sandy heavy clay layer – MD3

Samples

Bulk sediment

Coarse sands

Fine sands

Silts

Clays

Bulk sediment

Coarse sands

Fine sands

Silts

Clays

Bulk sediment

Coarse sands

Fine sands

Silts

Clays

Ref. code Smectite Kaolinite Illite Quartz Feldspars (+ Plagioclases) Hematite Goethite Ilmenite Amphibole Olivine Rutile Pyroxene Zircon

MD11 ++ ++ + ++ +

MD14 + + +++ ++

MD12 + + ++ +++ +

MD15 ++ ++ + + –

MD13 ++ ++ + + –

MD20 – ++ +++ +++ +

MD24 – – ++ +++ +

MD23 – – ++ +++ +

MD22 + ++ +++ + –

MD21 + +++ ++ + –

MD30 + ++ + ++ +

MD34 – – +++ ++

MD33 – – + +++ ++

MD32 ++ ++ ++ + –

MD31 +++ +++ ++ + –

– – – – + + – –

– – – – – + – –

– – + + + – – –

– – – – – – – –

– – – – – – – –

+ – + + – + + –

– – + + – + + –

– – + ++ – + + –

– – – – – – – –

– – – – – – – –

– + – + – + + –

– – + + – – – –

– – + + – + + +

– – – – – – – –

– – – – – – – –

+++: very abundant; ++: abundant; +: poorly represented; e: trace; –: not identified.

e

P.-D. Ndjigui et al. / Journal of African Earth Sciences 100 (2014) 164–178

in Fig. 1. Each sample shows different heavy minerals proportions with epidote and olivine dominating in the sandy clay layers, while opaque minerals increase in proportion from the lower to upper layers (Table 2). The bulk mineralogical compositions of samples reveal the occurrences of smectite, kaolinite, quartz, illite and few amounts of rutile, amphibole, olivine, pyroxene, feldspars, ilmenite, goethite and hematite (Table 3). The sand fractions show the same minerals as the bulk sediments but with a higher quartz proportion (Table 3). Note that amphibole amount may be abundant with some pyroxene and olivine in the sandy clay layers (MD1 and MD2). The most

abundant minerals in the finest sediments are kaolinite, illite and smectite (Table 3). Fig. 3 confirms the high proportion of kaolinite and illite in the oriented sub-samples of clay- and silt-sized fractions. Smectite mainly occurs in the fine fractions of the upper layer (Fig. 3e-f). 4.2. Geochemical characterization 4.2.1. Main geochemical patterns The main geochemical patterns of the Ngaye River alluvial sediments are first defined by a principal component analysis (PCA) of

b

32.5

2.5

22.5

32.5

d

2.5

12.5

12.5

22.5

32.5

Degree 2θ

f

2.5

illite illite + kaolinite

smectite

Intensity (arbitrary unit)

kaolinite illite + quartz

quartz

illite

illite + kaolinite kaolinite

smectite

Intensity (arbitrary unit) 2.5

kaolinite

illite + quartz

32.5

kaolinite illite + quartz

22.5

32.5

Degree 2θ

Degree 2θ

e

kaolinite

illite illite + kaolinite

smectite

Intensity (arbitrary unit)

kaolinite illite + quartz

quartz

kaolinite

12.5

22.5

Degree 2θ

kaolinite

2.5

illite illite + kaolinite

smectite

Intensity (arbitrary unit)

c

quartz

kaolinite

illite illite + kaolinite

12.5

Degree 2θ

12.5

22.5

illite + quartz

22.5

smectite

Intensity (arbitrary unit)

kaolinite illite + quartz

quartz

illite illite + kaolinite kaolinite

12.5

kaolinite

2.5

silt-sized fraction

quartz

a

smectite

n I tensity (arbitrary unit)

clay-sized fraction

quartz

168

32.5

Degree 2θ

Untreated

Treated with ethylene glycol

Treated by heating (500-600 °C)

Treated with hydrazine

Fig. 3. X-ray diffraction analyses of the finest grain-sized fractions after specific treatments (Left column: clay- and right column silt-sized fractions): (a and b) lower sandy clay layer; (c and d) Middle sandy clay layer; (e and f) upper sandy heavy clay layer.

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60

Factor 2 (39.3 % of total variance)

Ti-Zr heavy minerals 40

TiO2, Zr, Y, Nb, Hf Th, U,Ta, Cd, W La, Ce, Pr, Nd, Sm, Eu Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu

quartz + plagioclases

20

SiO2, CaO, Na2O Ba, Sr

0

Al2O3, Fe2O3, MnO, MgO, P2O5, H2O Cr, V, Ni, Zn, Cu, Co, Sc Li, Ga, Pb, Mo, Be, Cs, Tl

K2O, Rb K-felspars -20

clays + Fe-Mn oxihydroxides -40 -80

-60

-40

-20

0

20

40

60

80

100

Factor 1 (41.9 % of total variance) chemical elements sandy clay (MD1) bulk

sandy clay (MD2)

coarse sands

fine sands

sandy heavy clay (MD3) silts

clays

Fig. 4. Principal component analysis (PCA) diagram of the geochemical data, each sample being projected in the F1–F2 plan.

the geochemical compositions of the different grain-sized fractions of each sediment layer (Fig. 4). The first two factors F1 and F2 describe 41.9% and 39.3% of the total variance, respectively (Fig. 4). The projection diagram F1–F2 allows the discrimination of SiO2, CaO, Na2O (quartz, plagioclases) and K2O, Rb (K-feldspars) from TiO2, Zr, REE (Ti–Zr heavy minerals) and Al2O3, Fe2O3, MnO, MgO, P2O5, LOI and the other trace elements (clays and Fe–Mn oxihydroxides). The projection F1–F2 splits the four above mentioned geochemical groups in separate fields (Fig. 4). Based on this PCA analysis and the projection of the samples in the F1–F2 diagram, the different size fractions of the alluvial sediments are characterized by specific or a mixing of the geochemical signatures (Fig. 4). The silt- and clay-sized fractions of the Ngaye River sediments are relatively enriched in Al2O3, Fe2O3, MgO, P2O5, MnO and transition elements (V, Cr, Ni, Co, Cu and Zn) including also Sc, Pb, Li, Ga, Cs, Be and Mo, the silt fraction and the bulk of the lower sandy clay layer (MD1) are also imprinted by K2O and Rb. These fractions are also rich in kaolinite and smectite (Fig. 3, Table 3). All the coarse sand fractions with the fine sands of the lower layer (MD1) and the bulk samples of middle and upper layers (MD2 and MD3) are placed between quartz and plagioclases and K-feldspars. Ti, Zr, Hf, Th, U, Y and REE characterize the fine sand fractions of the middle and upper layers, and indicate a heavy mineral signature (Fig. 4).

4.2.2. Major chemical element patterns The Ngaye River sediments have moderate to high SiO2 contents (38.06–91.72 wt.%), with a mean of 64.89 wt.% SiO2 (Table 4). Al2O3 content increases as SiO2 decreases, the sand fractions being

more siliceous than the expected granitoid source rocks (BG, BMG and HBG) while the finest sediments are more aluminous (Fig. 5a). Ti and P are enriched in the silt- and clay-sized fractions, but Ti content is higher in the fine sand fraction of the upper sandy heavy clay layer (MD3) indicating the presence of rutile and/or ilmenite (Tables 3 and 4). The silt–clay and sand fractions have equivalent (CaO + Na2O + K2O) contents, but differentiated Al2O3 contents (Fig. 5b). A positive trend occurs for the sand fractions and the expected granitoid rocks source, while Al2O3 and (CaO + Na2O + K2O) are clearly negatively correlated for the silt– clay fractions (Fig. 5b), suggesting that feldspars are little weathered in the finest sediments and well preserved with quartz in the sand fractions (Fig. 5c). This moderate weathering of the source rocks agrees with limited depletion of Ca, K and Na relative to Al2O3 and Fe2O3 (Fig. 5b–d). MgO, Fe2O3 and the Chemical Index of Alteration (CIA) are negatively correlated with the SiO2/Al2O3 ratio (Fig. 5e–g), implying that Fe2O3 content increases with increasing weathering (Fig. 5h). In Fig. 5g, SiO2/Al2O3 comprised between 1.5 and 2.5 for CIA > 80% indicates that kaolinite and smectite (e.g., Mg-smectite, see Fig. 5 and Fig. 5g) are the most abundant clay minerals in the finest fractions (Table 3, Fig. 3).

4.2.3. Trace element distribution patterns Geochemical composition of trace element normalized to PAAS (McLennan, 1989) provide an overview of the geochemical patterns of the different grain-sized fractions in each layer (Table 5, Fig. 6a–c). The coarse sand fraction from the lower and middle layers is strongly depleted in U, Th, La, Ce, Nd, Sm and Yb, and slightly in Rb, Zr, Eu, Ca, Fe, Co and Cr (Fig. 6a). These two layers show

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Table 4 Major element composition (wt.%) and element ratios of Ngaye River sediments. Samples

d.l.

Lower sandy clay layer – MD1

Middle sandy clay layer – MD2

Fine sands MD12

Clays

–

Coarse sands MD14

Silts

Ref. code

Bulk sediment MD11

MD15

SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 LOI Total CIA (%) SiO2/Al2O3

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.05 – – –

55.05 19.80 7.16 0.07 1.21 1.04 0.92 1.78 0.82 0.07 11.16 99.08 84.11 2.78

83.27 6.91 2.35 0.02 0.79 0.72 1.14 1.86 0.29 0.08 2.46 99.89 65.00 12.05

74.02 12.65 2.81 0.03 1.07 2.14 2.47 2.36 0.44 0.05 1.92 99.96 64.48 5.85

48.01 18.79 11.05 0.11 1.90 1.06 0.85 2.22 1.03 0.58 13.51 99.11 88.41 2.55

Upper sandy heavy clay layer – MD3

Coarse sands MD24

Fine sands MD23

Silts

Clays

MD13

Bulk sediment MD20

Coarse sands MD34

Fine sands MD33

Silts

Clays

MD21

Bulk sediment MD30

MD22

34.71 22.97 12.77 0.12 1.22 0.56 1.22 1.23 0.99 3.32 19.50 98.61 81.98 1.51

75.04 10.89 4.25 0.04 0.85 1.07 1.08 2.00 0.48 0.04 3.87 99.62 72.41 6.89

91.72 4.61 0.62 0.01 0.10 0.58 0.77 1.49 0.06 0.02 0.34 100.31 61.88 19.89

85.45 6.25 2.28 0.05 0.67 1.13 0.77 1.29 0.74 0.06 0.39 98.98 66.21 13.67

43.31 21.94 12.46 0.12 1.73 0.59 0.64 1.97 1.18 1.17 14.35 99.46 87.27 1.97

MD32

MD31

38.06 23.49 12.58 0.09 1.20 0.40 0.97 1.34 1.07 2.63 17.62 98.94 89.66 1.62

67.96 13.12 5.56 0.05 0.64 0.83 0.73 1.85 0.64 0.06 7.50 98.94 79.37 5.17

89.76 4.92 1.02 0.01 0.12 0.67 0.99 1.47 0.21 0.03 0.34 99.55 61.12 18.24

81.29 6.17 4.30 0.09 0.50 1.31 1.05 1.35 2.17 0.19 0.50 98.91 62.45 13.17

46.12 21.78 10.47 0.10 1.18 0.72 0.46 1.72 1.04 0.53 14.70 98.82 88.25 2.12

41.26 25.14 11.37 0.09 1.10 0.57 0.35 1.08 0.99 0.79 16.45 99.19 92.63 1.64

d.l.: detection limits; LOI: Loss of ignition. CIA (%) = (Al2O3/(Al2O3 + CaO + Na2O + K2O))  100 from Nesbitt and Young (1984).

comparable trends but with a slight enrichment of Th, La, Ce, Nd, Zr and Sm and a large K negative anomaly in the fine sand fraction of MD2 (Fig. 6b). The upper sandy heavy clay layer (MD3) shows the same trend except the coarse fraction that is only depleted in Yb, Fe, Co, Cr (Fig. 6c). The Ngaye River sediments geochemical composition normalized to chondrite (McDonough and Sun, 1995) reveal that these sediments are strongly enriched in many elements except Fe, Co and Cr, and characterized by negative anomalies in K and Na (Fig. 6d–f). These anomalies and their chemical trends are concordant to the spider diagram forms of the expected granitoid rock source (Fig. 6g). Lithophile elements Y, Nb, Hf, Th, U, Ta, Cd and W are associated to TiO2 and Zr, i.e., to ilmenite/rutile in the residual sand fractions (Fig. 4). Y, Hf, Nb, and Ta contents are very low and Zr content varies between 14 and 5341 ppm (Table 5). However, the highest content of several trace elements are observed in the fine sand fraction of the upper layer (Table 5) in relation with heavy mineral concentrations. Transition elements with Li, Ga, Pb, Mo, Be, Cs and Tl are rather associated to Al2O, Fe2O3, MnO, MgO and P2O5 that characterized the silt and clay fractions (Fig. 4, Tables 4 and 5). Sr and Ba exhibit similar behavior and are linked to SiO2, CaO and Na2O, i.e., to the bulk sediment and to residual plagioclases concentrated in the sand fractions (Fig. 4, Tables 4 and 5). The concentration of transition trace elements such as Ni and Cr is low relative to Ba and Sr contents, although Cr, V, Ni, Zn, Co and Li have higher concentrations in the clay fractions (Fig. 7a, Table 5). Ba content varies from 230 to 1499 ppm, with highest concentration in the coarse sand fraction of the lower sandy clay layer (Table 5). Ba/Cr and Sr/Ni decrease as CIA increases suggesting the preferential retention of Ni and Cr in the finest grain-sized fractions (Fig. 7b and c). Zr, Ti and Th are relatively enriched in weathering profiles (Beauvais and Colin, 1993; Braun et al., 1993; Ndjigui et al., 2008, 2009) and also in sediments by heavy minerals sorting process. As such Zr and Th are used for testing the behavior of some trace elements. Regardless of its content in the expected source rocks, Zr increases in the sand fractions and decreases in the finest sediments (see Fig. 6a–c) with increasing Y content and CIA (Fig. 7d). Zr/Y increases with Th/Y (Fig. 7e), indicating that Th is preferentially hosted by zircon in the fine sand fractions (Table 5). Ni and Y increase with CIA relatively to Sr and Zr and are mainly

concentrated in the silts and clays (Fig. 7c–d, Table 5). These fractions are characterized by ratios Th/Y < 1, Zr/Y < 10 (Fig. 7e) and, Sr/Ni 6 1 and Y/Hf P 10 (Fig. 7f). 4.2.4. Rare-earth element fractionation Total REE content varies between 29 and 1851 ppm in the fine sands, with moderate concentrations (180–282 ppm) in the bulk sediments and silt–clay fractions (Table 6). La, Ce and Nd have higher contents than other REE, particularly in the fine sands. The total content of these three lanthanides vary between 21 and 1609 ppm, with less than 500 ppm in several samples (Table 6). Amongst HREE, only Gd and Dy have relatively high concentrations (>3 ppm) in the different size fractions. REE currently distribute between Ti and Zr heavy minerals (Fig. 4). HREE are rather carried by ilmenite, rutile and monazite. LREE are mostly carried by zircon and as previously mentioned Zr is highly concentrated in the fine sand fractions (Table 5). REE concentrations normalized to PAAS (McLennan, 1989) indicate: (i) REE-enrichment except in the coarse sands; (ii) slight LREE-enrichment and HREE-depletion; (iii) positive Eu anomalies (Eu/Eu 1.34 to 3.96), particularly for the coarse sand fractions of the sandy clay layers (Fig. 8a and b); (iv) large negative Eu anomalies (Eu/Eu 0.35 to 0.70) for the fine sand fractions of the middle and upper layers (MD2 and MD3) and the coarse sand fraction of the upper layer (Fig. 8b and c); (v) REE slight fractionation between the different grain-size (LaN/YbN varying between 0.72 and 7.37); and (vi) low (La/Sm)N and (Gd/Yb)N ratios (Table 6). Several samples have similar PAAS-normalized REE patterns with high abundances (Fig. 8a–c). The chondrite-normalized REE patterns confirm the similar high LREE-enrichment and Eu anomalies for studied samples and expected granitoid source-rocks (Fig. 8d–g). The fine sand fractions of the middle and upper layers (MD2 and MD3) have REE patterns comparable to those of BG and BMG (Fig. 8e–g), while the REE pattern of the finest fractions (silts and clays) rather underlines the signature of hornblende-biotite granitoids, HBG (Fig. 8d–g). HREE are concentrated in the finest fractions (silts and clays) rather than in the sand fractions (Fig. 9a–f, Table 6). The finest fractions are slightly depleted in LREE but relatively enriched in HREE that may be correlated to increasing Ti content and CIA in the same fractions (Fig. 9a and b). Conversely, the sand fractions are enriched in LREE relative to HREE (LREE/HREE 6.59 to 21.63). The highest

171

90

25

80

20

Al2O3, wt. %

SiO2, wt. %

P.-D. Ndjigui et al. / Journal of African Earth Sciences 100 (2014) 164–178

70 60 50

15 10

40

5

a 30

b 0

0

5

10

15

20

25

30

CaO+Na2O+K2O, wt. %

0

2

4

6

8

10

12

14

Al2O3, wt. % 15

Al2O3 / (CaO+Na2O+K2O)

Al2O3 / (CaO+Na2O+K2O)

15

10

5

c 0

30

40

50

60

70

80

90 100

10

5

Fe2O3, wt. %

d 00

5

10

15

5

10

15

20

20

15

15

SiO2/Al2O3

SiO2/Al2O3

SiO2, wt. %

10

10

5

5

0

e 0

0.5

1

1.5

2

2.5

3

3.5

f 0

0

Fe2O3, wt. %

MgO, wt. % 15

Fe2O3, wt. %

SiO2 / Al2O 3

20

15

10

10

5 5

g 0

55

60

65

70

75

80

85

90

95

h

0 55

CIA %

60

65 70

75

80

85 90

95

CIA % sandy clay (MD1) bulk sediments

sandy clay (MD2) coarse sands

sandy heavy clay (MD3) fine sands

silts

BMG

BG

HBG

clays

Fig. 5. Geochemical Harker diagrams of selected major elements: (a) SiO2 wt.% vs. Al2O3 wt.%; (b) Al2O3 wt.% vs. (CaO + Na2O + K2O) wt.%; (c) Al2O3/(CaO + Na2O + K2O) vs. SiO2 wt.%; (d) Al2O3/(CaO + Na2O + K2O) vs. Fe2O3 wt.%; (e) SiO2/Al2O3 vs. MgO wt.%; (f) SiO2/Al2O3 vs. Fe2O3 wt.%; (g) SiO2/Al2O3 vs. CIA%; (h) SiO2/Al2O3 vs. CIA% (HBG: hornblende-biotite granitoids; BG: biotite granitoids and BMG: biotite ± muscovite granitoids).

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Sample / PAAS

10

sandy clay (MD1)

1

10 -1

Sample / chondrite

10 4 10 3

sandy clay (MD1)

10 2 10 1 10 -1 10 -2

10 -2 Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

Sample / PAAS

10

sandy clay (MD2)

1

10 -1

d

Sample / chondrite

a

10 -3 Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

10 4

sandy clay (MD2)

10 3 10 2 10 1 10 -1 10 -2

Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

Sample / PAAS

sandy heavy clay (MD3)

10 1 10 -1

c

e

Sample / chondrite

10 -2

10 -3 Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

10 4

sandy heavy clay (MD3)

10 3 10 2 10 1 10 -1 10 -2

10 -2

10 -3 Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

bulk sediments coarse sands fine sands silts clays Biotite-muscovite granitoids (BMG)

f

Sample / chondrite

b

10 4

Granitoïds

10 3 10 2 10 1 10 -1 10 -2

Biotite granitoids (BG) Hornblende-biotite granitoids (HBG)

Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

g

10 -3 Rb U Th K La Ce Nd Zr Sr Na Sm Eu Yb Ca Fe Co Cr

Fig. 6. Geochemical spider diagrams of the Ngaye River sediments: (a–c) PAAS-normalized (McLennan, 1989); (d–g) Chondrite-normalized (McDonough and Sun, 1995).

LREE-enrichment in the fine sands is linked to the highest Zr content in the expected host granitoids, e.g., BMG (Fig. 9c). The highest HREE-enrichment in silt and clay fractions is linked to lowest Zr and highest Y contents characterizing the greenstones (BG and HBG). LaN/YbN varies from 18 to 80 in granitoids, and slightly fractionate around LaN/YbN 2 between the coarse and fine grain-

sized fractions that suggest REE fractionation with increasing weathering (Fig. 9d). The coarse sand fractions have higher Eu/Eu (3–4) than the finest sediments (Eu/Eu <1.5), which are characterized by GdN/YbN <2 (Fig. 9e). Granitoid signature is characterized by Eu/Eu <1.5, GdN/YbN P2, and CeN/YbN >1.5, which is potentially recorded by the fine sand fractions (Fig. 9e and f).

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P.-D. Ndjigui et al. / Journal of African Earth Sciences 100 (2014) 164–178

102

Ba/Cr

Ba/Cr

10 2

10

a

10

Sr/Ni

1 10-1

b

102

10

1

1 55

60

65

70

75

80

85

90

95

85

90

95

CIA % 10 2 10

2

Zr/Y

Sr/Ni

10 10

1 1

c

10 -1 65

60

70

80

75

85

90

d

95

55

60

65

70

75

80

CIA %

CIA % 102

10

Sr/Ni

Th/Y

10 1

1

e

Zr/Y

10-1

102

10

1

f

10-1 1

102

10

Y/Hf sandy clay (MD1)

sandy clay (MD2)

bulk sediments

coarse sands

BMG

sandy heavy clay (MD3) fine sands

silts

BG

HBG

clays

Fig. 7. Geochemical scattergrams of (a) Ba/Cr vs. Sr/Ni; (b) Ba/Cr vs. CIA%; (c) Sr/Ni vs. CIA%; (d) Zr/Y vs. CIA%; (e) Th/Y vs. Zr/Y; (f) Sr/Ni vs. Y/Hf (see Fig. 5 for HBG, BG and BMG acronyms).

5. Discussion Variations of the mineralogical, geochemical composition and sorting of the alluvial sediments of the Ngaye River document the source rock types including their weathering degree. The distribution of chemical elements in the bulk sediment and the different grain-sized fractions and the relatively low CIA values suggest sediment sources from the lower less altered parts (arenaceous saprolites) of weathering profiles developed mainly upon granitoid rocks (e.g., Fig. 5a–d). The geochemical patterns displayed in

Fig. 7 further suggest that these granitoid rocks are the main sources of the Ngaye River alluvial sediments. However, some transition elements and REE patterns may also suggest to a certain extent a potential origin of the sediments from greenstones and/ or basic volcanics (e.g., Figs. 6d–g, 8d–g and 9e–f). 5.1. Source rock geochemical signature of alluvial sediments Chemical element distribution in alluvial sediments depends on their grain size and mineralogical composition (Shimizu and

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Table 5 Trace element composition (ppm) and element ratios of Ngaye River sediments. Samples d.l.

Lower sandy clay layer – MD1

Ref. code

–

Bulk sediment MD11

Cr V Ni Zn Cu Co Sc Ba Zr Sr Y Li Ga Pb Rb Nb Hf Mo Th U Ta Be Sn W Tl

3.00 0.80 1.60 7.00 1.40 0.13 1.10 0.80 6.00 0.60 0.05 0.40 0.04 0.60 0.23 0.03 0.14 0.08 0.02 0.01 0.02 0.04 0.16 0.05 0.01

135.00 116.00 73.20 72.00 49.50 24.85 14.50 947.90 149.00 147.20 17.77 15.90 26.01 17.20 41.73 11.71 3.84 0.78 14.51 1.21 0.61 1.72 1.52 0.38 0.33

Middle sandy clay layer – MD2

Coarse sands MD14

Fine sands MD12

Silts

Clays

MD15

MD13

Bulk sediment MD20

45.00 30.50 29.20 35.00 26.00 12.81 3.40 982.10 85.00 152.00 2.18 6.10 7.96 9.60 43.37 3.27 2.14 0.33 1.65 0.21 0.18 0.44 0.54 0.31 0.19

72.00 101.00 160.00 68.00 45.40 95.70 95.60 68.70 31.10 55.10 56.00 27.70 38.00 63.00 60.00 40.00 23.30 38.50 118.40 20.60 11.84 19.00 17.54 14.31 6.30 13.20 14.20 8.30 1499.20 397.50 314.90 899.30 299.00 56.00 14.00 321.00 318.10 47.20 30.50 144.60 8.65 15.56 16.09 11.70 5.30 18.20 19.30 5.30 14.36 19.77 18.71 12.95 14.20 11.90 12.00 13.20 49.54 53.72 44.19 44.83 5.56 7.99 3.01 10.26 7.36 1.08 0.29 8.50 0.26 15.85 19.55 0.69 14.31 9.47 10.45 17.12 1.17 0.85 0.87 1.41 0.35 0.34 0.11 0.84 0.73 1.32 1.35 0.60 0.54 1.45 1.26 0.84 0.31 0.28 0.22 0.40 0.21 0.28 0.23 0.26

Upper sandy heavy clay layer – MD3

Coarse sands MD24

Fine sands MD23

Silts

Clays

Bulk sediment MD30

MD22

MD21

15.00 11.00 5.10 8.00 7.30 1.95 2.40 792.90 135.00 118.80 2.13 1.70 4.21 9.40 27.40 1.43 3.60 0.14 2.39 0.34 0.11 0.29 0.21 0.08 0.11

47.00 13.20 12.00 20.00 10.10 5.03 7.80 674.90 901.00 112.70 24.73 1.70 7.27 11.10 23.02 16.50 24.65 0.24 62.83 4.28 1.71 0.41 0.68 0.78 0.10

165.00 184.10 83.90 109.00 96.10 42.36 14.10 380.50 79.00 41.70 21.70 14.80 30.02 20.30 11.50 18.93 2.20 3.05 11.93 1.49 1.15 1.38 2.13 0.53 0.56

157.00 80.00 188.30 86.60 89.70 37.60 108.00 43.00 101.30 29.10 32.13 16.43 20.30 10.10 365.70 853.90 56.00 330.00 27.20 133.30 36.90 17.19 21.90 8.40 31.38 16.05 20.20 15.00 34.57 36.90 17.58 10.02 1.53 9.15 4.06 0.85 18.68 13.48 1.31 1.47 0.95 0.61 1.73 0.90 2.33 1.00 0.52 0.40 0.48 0.29

Coarse sands MD34

Fine sands MD33

Silts

Clays

MD32

MD31

36.00 15.80 7.30 16.00 14.20 2.21 2.40 912.90 246.00 155.60 5.94 1.80 4.56 9.00 25.73 2.32 5.95 0.12 19.99 1.20 0.17 0.33 0.22 0.09 0.10

93.00 152.00 164.00 71.10 167.50 169.70 15.60 78.40 89.60 36.00 88.00 95.00 16.30 72.10 69.60 6.49 33.53 35.07 6.90 21.20 19.00 846.90 712.70 230.40 5341.00 83.00 44.00 174.10 91.40 37.60 53.49 26.46 15.92 1.60 22.90 25.80 8.07 29.22 32.39 17.80 20.00 17.10 22.22 72.22 17.83 31.87 16.96 16.03 >29 2.27 1.25 0.45 1.87 2.19 224.00 15.39 12.68 15.20 1.40 1.10 2.81 0.96 0.90 0.39 1.69 1.85 0.84 1.81 2.02 1.38 0.54 0.51 0.09 0.48 0.45

d.l: detection limits.

Masuda, 1977; Sawyer, 1986; Kasanzu et al., 2008; Bhuiyan et al., 2011). As such, quartz-rich alluvial sediments may result from the erosion of large tracts of granitoids in the Ngaye River watershed. The high Ba and Sr contents (Fig. 7a and b) are indicative of feldspars (including plagioclases) depending on the granitoid nature of source rocks (Tchameni et al., 2006). Mg, Cr and Sc contents are rather indicative of ferromagnesian minerals such as biotite, pyroxene, amphibole and olivine (Nesbitt, 1979), and chromite, which are current minerals of greenstones (see Fig. 1b). High proportion of quartz in the alluvial sediments may explain the low trace element contents (including REE) in the coarse sand fractions (Das et al., 2006; Tchameni et al., 2006). The overall REE distribution in the Ngaye River sediments reflects the REE distribution in the granitoid rocks (Fig. 8a–g). The moderate REE contents in the clay-sized fractions depend on clay mineral species. The LREE-enrichment may be linked to Zr and/or epidote (Gromet and Silver, 1983), which are concentrated in the sand fractions of the upper sandy heavy clay layer (Fig. 8b, Tables 2 and 5). Large Eu positive anomaly (Eu/Eu 3.2 to 4) of the coarse sand fractions of the sandy clay layers (Figs. 8d–e and 9e), might be potentially a signature of the andesitic basalts located in the southwest part of the Ngaye River watershed (Fig. 1a). Large positive Eu anomaly was also described in mafic Tertiary volcanics from Northwest Cameroon (Temdjim et al., 2004; Kamgang et al., 2013). REE distribution patterns of PanAfrican granitoids however show a negative Eu anomaly (Fig. 8g) suggesting that these rocks are potentially the main sources of the Ngaye River sediments. The Eu anomalies might also reflect differential enrichment by mineral sorting of resistant heavy minerals (e.g., ilmenite, rutile) and/or moderately weathered feldspars (Leybourne et al., 2006; Lee et al., 2009) or differences in weathering degree of the inherited primary mineral assemblage (e.g., feldspars, plagioclases and Fe–Mg minerals) in the sedimentary layers.

5.2. Sorting and weathering of alluvial sediments Sorting of alluvial sediment determines the mineral distribution in the different grain-sized fractions that also primarily depends upon the composition and weathering degree of source rocks. Unaltered feldspars and pyroxene attest of the sediment immaturity due to short distance transport from moderately weathered source rocks (Hofmann, 2005; Das et al., 2006; Marques et al., 2011). Illite is inherited from such source rocks as well as kaolinite and smectite. These clay minerals may have also neo-formed by weathering process of primary minerals in the alluvial sediments. For example, smectite may derive from the chemical weathering of residual Fe–Mg minerals. Weathering and sorting of minerals, such as feldspars and/or amphibole and pyroxene control Ba and Sr content variations (Das et al., 2006; Rahman and Suzuki, 2007; Harlavan et al., 2009; Bhuiyan et al., 2011), relatively to the transition elements, e.g., Ni and Cr, in the different grain-sized fractions (Fig. 7a–c). Chemical weathering process in sediments currently removes labile cations, e.g., Ca2+, Na+, K+, and concentrates relatively stable residual constituents (Fedo et al., 1995), e.g., Fe, Al, Ti. Limited depletion of Na, Ca and K are linked to the moderate weathering of feldspars (including plagioclases), while high Al2O3 content indicates high proportion of micas and clays (Das et al., 2006; Marques et al., 2011). Highest concentrations of CaO in the sand fractions (e.g., fine sands) may also be attributed to accessory mineral concentrates (e.g., epidote, see Table 2). Moderate TiO2 and Fe2O3 contents in sands could be related to ilmenite (Zhou et al., 2004). Decreasing SiO2/Al2O3 ratio, MgO content and increasing Fe2O3 content (Fig. 5e–h) may also reflect the differentially weathered residual Fe–Mg minerals such as olivine and/or amphibole and pyroxene, which are inherited from moderately weathered granitoids. High Fe2O3 content may also reflect the formation of Fe–Mg clays such as smectite in the finest sediment grain-sized fractions and in a lesser degree Fe-oxihydroxides.

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a

sandy clay (MD1)

1

10 -1

10 -2 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sample / PAAS

sandy clay (MD1) 102

10

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

103

10

b

d

sandy clay (MD2)

1

10 -1

sandy clay (MD2)

Sample / chondrite

Sample / PAAS

10

Sample / chondrite

103

10 -2

102

10

e

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sample / PAAS

c

sandy heavy clay (MD3)

10

1

Sample / chondrite

104 10 2

sandy heavy clay (MD3) 103

102

10

10 -1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

f

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

bulk sediments coarse sands fine sands silts clays Biotite-muscovite granitoids (BMG)

Sample / chondrite

103

Granitoïds

102

10

Biotite granitoids (BG) Hornblende-biotite granitoids (HBG)

g

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 8. REE patterns of the Ngaye River sediments: (a–c) PAAS-normalized (McLennan, 1989); (d–g) Chondrite-normalized (McDonough and Sun, 1995).

6. Conclusion The Ngaye River alluvial sediments result from the erosion of moderately weathered proximal granitoids with a marked

greenstone signature but a contribution from basalts may not be totally dismissed. Three sedimentary layers were differentiated by their granulometric texture: sandy clay and sandy heavy clay layers. The mineral assemblages characterizing the different

P.-D. Ndjigui et al. / Journal of African Earth Sciences 100 (2014) 164–178

35

35

30

30

25

LREE / HREE

LREE / HREE

176

20 15 10

25 20 15 10

5

a

Ti/Zr

0 1

10

5

b

102

55

60

65

70

75

80

85

90

95

85

90

95

CIA % 35

102

LaN / YbN

LREE / HREE

30 25 20

10

15 1

10

c

Zr/Y

5 1

d

102

10

55

60

65

70

75

80

CIA % 8 4

7

CeN / YbN

6

Eu/Eu*

3

2

5 4 3 2

1

1

e

0 0

1

2

3

4

5

f

6

GdN / YbN

0 0

1

2

3

4

5

6

GdN / YbN sandy clay (MD1)

sandy clay (MD2)

bulk sediments

sandy heavy clay (MD3)

coarse sands

fine sands

BMG silts

BG

HBG

clays

Fig. 9. Geochemical scattergrams of (a) LREE/HREE vs. Ti/Zr; (b) LREE/HREE vs. CIA%; (c) LREE/HREE vs. Zr/Y; (d) LaN/YbN vs. CIA%; (e) Eu/Eu vs. GdN/YbN; (f) CeN/YbN vs. GdN/ YbN (see Fig. 5 for HBG, BG and BMG acronyms).

grain-sized fractions of these layers are quartz/feldspars (+ heavy minerals) in the sand fractions, and kaolinite/smectite (+ illite) in the finest sediments. This mineralogical sorting controls the geochemical fractionation of trace and rare-earth elements, with an obvious dependence on the mineral assemblages including the weathering degree of minerals in the different grain-sized fractions. Transition elements (e.g., V, Cr, Ni, Co, Cu and Sc) are relatively enriched in the most weathered silt- and clay-sized fractions, which are the richest in clay minerals (kaolinite, illite and smectite). Lithophile elements (e.g., Na, Ca, Ba, Sr and to a lesser extent K) and Zr are rather concentrated in the less weathered sand

fractions, which are the richest in quartz, moderately altered feldspars and heavy minerals. Chemical weathering fractionates REE between the different grain-sized fractions, with highest REE and LREE contents in the less weathered finest sand fractions, while lowest lanthanide and highest HREE concentrations are measured in the most weathered and finest grain-sized fractions. Acknowledgements The authors gratefully acknowledge the experience and field assistance of Milan Tchouatcha and Elisé Sababa of the University of Yaoundé 1, Cameroon. Our fieldwork was facilitated by the

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P.-D. Ndjigui et al. / Journal of African Earth Sciences 100 (2014) 164–178 Table 6 Rare-earth element composition (ppm) and element ratios of Ngaye River sediments. Samples

d.l.

Lower sandy clay layer – MD1

Ref. code

–

Bulk sediment MD11

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE LREE HREE LREE/HREE La + Ce + Nd a Ce/Ce Eu/Eu (La/Yb)N (La/Sm)N (Gd/Yb)N

0.040 48.35 0.120 99.70 0.010 10.16 0.060 36.61 0.010 6.07 0.003 1.49 0.010 4.50 0.002 0.62 0.009 3.55 0.040 0.67 0.007 1.90 0.001 0.27 0.009 1.72 0.002 0.26 – 215.87 – 202.58 – 13.29 – 15.24 – 184.66 – 0.86 – 1.04 – 1.34 – 2.08 – 1.16 – 1.59

Middle sandy clay layer – MD2

Coarse sands MD14

Fine sands MD12

Silts

Clays

Bulk sediment MD20

MD15

MD13

6.98 13.18 1.44 5.30 0.79 0.45 0.57 0.07 0.39 0.08 0.22 0.03 0.25 0.04 29.79 28.14 1.65 17.06 25.46 0.86 0.96 3.16 2.08 1.28 1.39

38.26 69.49 7.97 28.18 4.47 0.95 3.18 0.37 1.85 0.33 0.90 0.14 0.92 0.14 157.15 149.33 7.83 19.07 135.93 0.87 0.92 1.19 3.07 1.24 2.09

43.88 47.83 42.81 81.12 98.27 84.44 8.85 9.55 9.35 31.09 33.76 33.42 5.06 5.27 5.68 1.27 1.29 1.04 3.93 3.98 4.11 0.53 0.53 0.50 2.93 3.07 2.54 0.56 0.60 0.46 1.52 1.65 1.24 0.21 0.24 0.18 1.34 1.47 1.15 0.20 0.21 0.18 182.15 207.73 187.10 171.27 195.97 176.74 11.24 11.76 10.36 15.24 16.66 17.06 156.09 179.86 160.67 0.86 0.87 0.86 0.95 1.06 0.94 1.34 1.33 1.01 2.41 2.40 2.75 1.26 1.32 1.10 1.77 1.64 2.16

Upper sandy heavy clay layer – MD3

Coarse sands MD24

Fine sands MD23

Silts

Clays

Bulk sediment MD30

MD22

MD21

5.92 10.54 1.23 4.30 0.77 0.46 0.58 0.08 0.43 0.08 0.23 0.04 0.25 0.04 24.93 23.22 1.71 13.58 20.76 0.83 0.90 3.96 1.74 1.12 1.39

131.26 250.79 27.88 99.09 16.21 1.30 11.11 1.30 6.00 0.97 2.37 0.31 1.97 0.31 550.87 526.53 24.34 21.63 481.14 0.87 0.96 0.46 4.92 1.18 3.41

25.96 59.54 6.67 25.31 4.80 1.25 4.12 0.65 4.27 0.89 2.63 0.40 2.60 0.39 139.48 123.53 15.95 7.74 110.81 0.79 1.04 1.32 0.74 0.79 0.96

63.15 40.44 126.42 81.45 12.85 9.17 45.53 33.62 8.14 5.62 2.16 1.27 6.71 4.13 1.05 0.58 6.49 3.36 1.30 0.66 3.70 1.84 0.53 0.27 3.28 1.76 0.47 0.27 281.78 184.44 258.25 171.57 23.53 12.87 10.98 13.33 235.10 155.51 0.83 0.84 1.02 0.98 1.38 1.24 1.42 1.70 1.13 1.05 1.24 1.42

Coarse sands MD34

Fine sands MD33

Silts

Clays

41.39 78.93 8.80 30.70 4.83 0.57 3.06 0.34 1.50 0.24 0.57 0.07 0.46 0.07 171.53 165.22 6.31 26.18 151.02 0.88 0.95 0.70 6.65 1.25 4.03

442.19 53.75 15.60 850.97 107.64 31.44 89.05 11.38 3.37 316.09 40.26 12.39 50.35 6.78 2.67 2.98 1.74 0.80 32.57 5.26 2.55 3.52 0.77 0.43 14.66 4.77 2.77 2.15 0.95 0.56 4.86 2.75 1.68 0.66 0.40 0.24 4.43 2.52 1.59 0.78 0.38 0.24 1851.26 239.35 76.33 1751.63 221.55 66.27 63.63 17.80 10.06 27.53 12.66 6.59 1609.25 201.65 59.43 0.87 0.84 0.78 0.99 1.00 1.00 0.35 0.88 1.44 7.37 1.57 0.72 1.28 1.15 0.85 4.45 0.72 0.97

MD32

MD31

d.l. detection limits. a = (La + Ce + Nd)/REE. Ce/Ce = (Cesample/CePAAS)/(Lasample/LaPAAS)1/2 (Prsample/PrPAAS)1/2. Eu/Eu = (Eusample/EuPAAS)/(Smsample/SmPAAS)1/2 (Gdsample/GdPAAS)1/2. (La/Yb)N = (Lasample/LaPAAS)/(Ybsample/YbPAAS). (La/Sm)N = (Lasample/LaPAAS)/(Smsample/SmPAAS). (Gd/Yb)N = (Gdsample/GdPAAS)/(Ybsample/YbPAAS).

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