Vertical distribution of trace elements in laterite soil (Suriname)

Chemical Geology, 47 (1984/1985) 159--174 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

VERTICAL DISTRIBUTION ( S U R , I N A M E ) .1

159

O F T R A C E E L E M E N T S IN L A T E R I T E SOIL

S.C. TOPP',*~ , B. SALBU' , E. ROALDSET 2,.3 and P. JORGENSEN 3

' Department o f Chemistry, University o f Oslo, Oslo 3 (Norway) ~"Department o f Geology, University o f Oslo, Oslo 3 (Norway) 3 Department o f Geology, Agricultural University o f Norway, 1432-,~s-NLH (Norway) (Received April 26, 1983; revised and accepted January 18, 1984)

ABSTRACT Topp, S.E., Salbu, B., Roaldset, E. and J6rgensen, P., 1984. Vertical distribution of trace elements in laterite soil (Suriname). Chem. Geol., 47: 159--174. The vertical distribution of 20 elements in a 12.5-m thick laterite profile from Suriname, South America, has been investigated using neutron activation analysis, atomic absorption spectrometry and X-ray fluorescence. The mineralogical composition was quantitatively determined from calculations based on results from X-ray diffraction, thermal gravimetric methods and from the data obtained for major elements using X-ray fluorescence. The most characteristic change observed in the mineralogy with depth is the increase in kaolinite relative to gibbsite. A considerable fractionation with depth is found for different trace elements. The enrichment of the lanthanoids La and Ce at 3--4 m below surface, and of Sm and Eu at 10--12.5-m depth, demonstrates that the distribution of elements, despite the similarity in chemical properties, is seriously affected by weathering processes. The laterite soil samples were also separated in A1- and Fe-rich fractions using NaOH. These separation experiments demonstrated that the trace elements are mainly associated with the Fe-rich (red mud) fraction of the laterite soil.

INTRODUCTION

B a u x i t e s a n d l a t e r i t e s are t h e u l t i m a t e p r o d u c t s o f i n t e n s e a n d l o n g lasting chemical weathering of crystalline rocks in humid tropical regions. Their formation involves some of the most extreme geochemical separation p r o c e s s e s k n o w n i n n a t u r e . T h e p h e n o m e n o n is o f g r e a t g e o c h e m i c a l int e r e s t a n d t h e d e p o s i t s are o f f u n d a m e n t a l e c o n o m i c i m p o r t a n c e f o r m a n y countries. However, surprisingly few investigations o n the t r a c e - e l e m e n t *~ Work based on a thesis by Sven Erik Topp. Present addresses: *: Kvaerner Engineering A.S., Box 222, 1324 Lysaker, Norway. .3 Norsk Hydro A.S., Research Centre, Lars Hillesgt. 30, 5000 Bergen, Norway.

0009-2541/84-85/$03.00

© 1984 Elsevier Science Publishers B.V.

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Bauxite

Tertiary Coarse sand, common[y strongly leached

Holocene and Pleistocene J S a n d s , s i l t y clays and c l a y s

LEGEND

•v••l• SURINAME

~ig. 1. Simp]it'ied geological map of" ~orl, hwes~ Suriname. ~he prot'i]e site is located in the Bakbuys ~V~ountains. inset map shows ]ocal;io~ oE Suriuame.

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161 distributions have been reported for lateritic soils (Balasubramaniam, 1978; Sahasrabudhe, 1978). Geochemical investigation of karstic bauxite deposits have, however, revealed high concentrations and occasionally extreme distribution patterns for multivalent metals, e.g., REE, Hf, V and Mn (MaksimoviS, 1976; Maksimovid and Roaldset, 1976; MaksimoviS, 1978). Although the karstic bauxite represents a more advanced stage of weathering than that of lateritic soils, similar fractionation processes are considered to be important for both. Therefore, in order to contribute to the understanding of geochemical differentiation of trace elements during lateritization, samples from a vertical profile through lateritic/weathered crust (duricrust) in Suriname have been selected for investigation. Geological setting

Suriname is located in the NE part of South America within the Guyana Shield (Fig. 1). The profile investigated lies on the Bakhuys Mountain horst. The bedrock here belongs to the oldest part of the Middle Precambrian (Early Proterozoic) Fallawatra Group (1900- 2500 Ma), and consists of charnockitic granulite, gneiss and amphibolite (Bosma et al., 1977; Aleva, 1979a, b). During Cretaceous to Recent times, several peneplanation surfaces developed, with contemporaneous lateritization and duricrust formation. In Suriname, remnants of an Early Tertiary surface are associated with laterites which cap the flat inland plateaus and shelter them from erosion ("plateau bauxites") (Krook, 1975). In the coastal-plain sediments, a corresponding hiatus -- "the bauxite h i a t u s " is distinguished and dated palynologically as Late Eocene--Oligocene (Krook, 1975; Aleva, 1979a, b). The lateritic and bauxitic weathering events on the Guyana Shield (Mendoza et al., 1978) vary from small occurrences of local interest to bauxite deposits of commercial value. The S u r i n a m e profile ~ w e a t h e r e d crusts

Lateritic and bauxitic soils form under tropical to subtropical conditions. The essential requirement for their formation appears to be high rainfall, intense leaching and a strongly oxidizing environment. Under such conditions, organic matter accumulation is inhibited. The leaching solution tends to flush out mobile constituents, leaving a residue of Al-, Ti- and ferric oxide/hydroxides (Keller, 1957; Loughnan, 1969). A deep weathered profile consists of a number of zones. As weathering proceeds downward from the surface, progressive subzones develop. Divalent Fe may migrate upward with pore water from a bleached clay zone to the surface horizon where it oxidizes to goethite. The lowest part of the profile passes gradually into the unweathered parent rock.

WEATHERED

CLAY

ALUMINOUS LATERITE BAUXITES ( PALLID )

ROCK

plastic when wet

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Fig. 2. Vertical s e c t i o n t h r o u g h a laterite d e p o s i t , S u r i n a m e . T h e 25 s a m p l e s i n v e s t i g a t e d were c o l l e c t e d at 0.5-m intervals. Mineralogical c o m p o s i t i o n w i t h d e p t h , as r e c a l c u l a t e d f r o m X-ray d a t a a n d c h e m i c a l analyses, is i n c l u d e d in t h e figure.

D

HORIZON

RESIDUAL SOIL ACCUMULATION ZONE hard : DURICRUST

NAME

b~

163 During weathering of crystalline rocks, the rock-forming minerals are partly dissolved; new minerals like illite and smectite are the earliest to form, followed by halloysite and kaolinite. In the final stage, as the leaching intensifies, partial desilification occurs and kaolinitic clay is converted to gibbsite. As the rock-forming minerals break down, the trace elements are liberated from the dissolving minerals and moved by runoff and percolating water. Since the weathered mantle acts as an ionic exchange column, repeated eluviations and ion-exchange processes will give rise to fractionation among elements. Fractionation among lanthanoids by natural processes is assumed to have taken place during formation of the Russian Platform sediments, the most extreme fractionation being associated with geological periods of humid climate and intense chemical weathering (Balashov et al., 1964). In weathering profiles on crystalline rocks, Ronov et al. ( 1 9 6 7 ) r e p o r t accumulation of the lightest lanthanoids at neutral to moderately alkaline conditions. The intermediate and heavier elements may accumulate at higher pH-values deeper in the profile. Absorption of lanthanoids to minerals and soils is well known (Aagaard, 1974). Sahasrabudhe (1978), investigating the chemical distribution of some trace elements in lateritic bauxites from India, reported multivalent elements (e.g., Ti, Mn, Cr, V, Ni, Ga, Cu) following the pattern of A1 and Fe in the bauxite cap, thus being strongly enriched in the weathered cap relative to the underlying bedrock.

The Suriname profile The profile section (Fig. 2) closely follows a standard laterite profile (Lelong, 1969; Aleva, 1979a, b) with the following characteristic sections developed (truncated profile): (1) An upper Fe-rich gossan, pisolitic (1.5 m) (2) Fe-rich lateritic bauxite (ferruginous) (1 m) (3) Aluminous lateritic bauxite (8--10 m) (4) Weathered rock (5) Unweatherea, or slightly weathered, rock The ultimate products of the weathering processes are mainly gibbsite, hematite, goethite and kaolinite. Hematite prevails in the surface lateritic crust. The 25 samples investigated in the present work were collected at 0.5-m intervals. In these samples, major elements (Si, A1, Ti, Fe) and trace elements (Na, Mg, Sr, Sc, La, Ce, Sm, Eu, Tb, Tm, Hf, Cr, Mn, Co, Ni, Zn) have been determined using different methods of analysis. In addition to determination of the total concentration, the distribution of the elements in red mud (Fe-rich) and the aluminous fraction also has been investigated. In order to elucidate the distribution of elements, mineralogical analyses have been included in the analytical programme.

164 EXPERIMENTAL METHODS

Sample preparation Laterite soil samples representing 0.5-m intervals through a 12.5-m vertical profile were provided by /~rdal & Sunndal Verk A/S. The samples were collected from drilled cores for prospecting purposes. Prior to analysis, samples of ~ 1--5 g were ground for 15 min. in a plastic mill containing distilled water.

Loss on ignition (LOI) About 100 mg of sample was dried at 110°C, accurately weighed, and loss on ignition was determined after heating the dried material for 2 hr. at 105O°C.

X-ray diffraction (XRD) Finely ground particles in suspension were sucked onto 0.2-~m filters (Millipore ®), inverted to glass slides and subjected to X-ray analysis. A Philips ® diffractometer with Cu-K~ radiation and Ni filter was used. The minerals were identified according to the criteria given by Brown (1961).

Thermal gravimetric methods (DTA, DTG, TGA) In order to obtain further information on the mineralogical composition, differential thermal analysis (DTA), differential thermal gravimetry (DTG) and thermal gravimetric analysis (TGA) were performed. About 30 mg of finely ground sample was accurately weighed and then heated in an aluminium crucible from 25 ° to 700°C in a nitrogen atmosphere. A Mettler ® TAI apparatus was used, with Type NT oven which has a heating rate of 4°C rain. -1.

X-ray fluorescence (XRF) Major elemental analysis (A1, Si, Ti, Fe) was performed at _~rdal & Sunndal Verk using a Siemens ® SRS-1 XRF spectrometer with standard curves based on international rock standards (V. Wiik, pers. commun., 1978).

Atomic absorption spectrometry (AAS) Dry Li2B407 was added to 0.200-g dry sample powder. The mixture was transferred to a graphite crucible before heating for 1 hr. at 1000°C. After cooling, the glass pearl was dissolved in 70 ml of 4 N HNO3. Nine elements

165 were d eter min ed in diluted fractions of the dissolved sample using a PerkinElmer ® 503 instrument with an a c e t y l e n e - a i r flame. Only selected samples from the profile were subjected to AAS. The elements A1, Si, Fe, Na, Mg, Mn, Ni and Zn were then determined.

Neutron activation analysis (NAA) Ab o u t 50 mg of finely crushed sample was transferred to an A1 envelope before accurately weighing. Samples of BCR-1 and BX-N, standard rock basalt and bauxite, respectively (U.S. Geological Survey), and the laterite samples were irradiated for 3 days in the reactor Jeep II, Kjeller, Norway, at a flux o f 1.5.1013 n s -1 cm -2. After a decay time of 3 days, the samples were transferred to a Teflon ® beaker for chemical separation.

Separation procedure In order to investigate the distribution of trace elements, especially lanthanoids, in the red mud (Fe-rich) and white (Al-rich) fractions, 50 ml of c o n c e n t r a t e d NaOH was added to the sample and the mixture was kept in a boiling water bath for 2 hr. The NaOH solution was then decanted into counting bottles and the dissolved A1 fraction was subjected to 7-spectrometry. No further significant dissolution of A1 was obtained by repeating the NaOH treatment. Ab o u t 5 ml of concent r a t e d HF and 5 ml of c o n c e n t r a t e d HNO3 were added to the residue and the mixture was allowed to dry. This procedure was repeated twice and then 10 ml of 9 M HC1 and 1 ml of 2% H3BO4 were added. After ~ 2 hr, the Fe-rich residue was dissolved and the solution was subjected to 7-spectrometry. A Canberra ® Ge(Li) d e t e c t o r with 20% efficiency and a resolution of 1.89 keV for the 1332-keV peak of 6°Co was used. Peak locations and calculation of peak areas were p e r f o r m e d according to the program GAMANAL (Gunnink et al., 1967), as described in the work of Salbu et al. (1975). The elements Fe, Sc, La, Ce, Sm, Eu, Tb, Tm, Hf, Cr and Co were then determined.

Quantitative mineralogical composition Mineralogy based on XRD is only semi-quantitative. Therefore calculations based on various analytical results from XRD, DTA, DTG, TGA and XRF were performed. The LOI values are assumed to be the result only of vaporization o f water and d e h y d r o x y l a t i o n , uninfluenced by oxidation of Fe which is present pr e dom i nant l y in the trivalent state. The principle of the calculations is illustrated in Fig. 3 for samples where kaolinite is identified and the quartz c o n t e n t is low. In samples where kaolinite is below the d e tectio n limit, all Si is assumed to be present as quartz.

166 ShEM[CAL C:)MPONETIT'

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CALCULATED M:~RALS

Fig. 3. The principles for mineralogical calculations based on semi-quantitative mineralogy (X-ray diffraction) combined with bulk chemical analyses.

RESULTS AND DISCUSSION

Mineralogy The mineralogical composition of the lateritic profile is calculated on the basis of data from XRD, TGA, DTG and XRF, and is included in Fig. 2. According to X-ray diffractograms, the following minerals have been identified: gibbsite (4.85 A), quartz (4.26 £ , 3.34 £ ) , hematite (2.69 A), goethite {4.21 £ ) and kaolinite (7.0 £ , 3.57 A). From the thermograms obtained, the minimum observed around 300°C reflects the dehydration of gibbsite. Some dehydration values are given in Table I. The water losses as estimated from the TGA curves (450°C) for the investigated samples vary within 14--24%. LOI-values determined at 1050°C (~rdal & Sunndal Verk) are somewhat higher, as expected, and range from 23% to 31%. Based on mineralogical calculations, the profile has the following characteristics. Gibbsite amounts to 54- -90% of the minerals present and is the predominant constituent throughout the profile. In the upper parts, gibbsite reaches 50%, goethite and limonite ~ 30--40%, and quartz up to 8% is also present.

167 In the intermediate part of the profile, the gibbsite content reaches its maximum of ~ 90%, and hematite its minimum of 1.4%. Near the base, kaolinite and hematite increase, but gibbsite still predominates. According to the mineralogical calculations, excess A1203 and TiO2 should be present at certain levels, probably as amorphous A1--Fe compounds. Although corundum and anatase/rutile are occasionally observed in lateritic soils (Deer et al., 1967; B~rdossy and PantS, 1971) neither corundum nor anatase/rutile could be detected by XRD in this work. The calculated mineralogical variations are in accordance with previous investigations of lateritic soils and represent the endpoints of chemical weathering (Chesworth, 1973, 1975).

Elemental composition The elemental composition of the laterite soil samples obtained by XRF, AAS and NAA is given in Table I, together with the corresponding data for basalt BCR-1 and bauxite BX-N, both from the U.S. Geological Survey. The mineralogical similarity of BX-N with the lateritic soil, however, made this standard preferable. As the data on BX-N are scarce, the composition of the elements of interest was determined using BCR-1 as a standard (Table I). The precision of the methods as given in the Table I is based on analysis of 3--4 replicates. For NAA, the precision also depends on counting statistics and is lowered as the determination limits are reached. Information on the accuracy of the methods used may be obtained by comparing the results of Fe, A1 and Si given in Table I. The data obtained by XRF seem to be somewhat high, but this will not significantly influence the mineralogical calculations.

Major elements In the investigated profile, the major-element oxides range within: SiO 2

TiO~ Al~O~ Fe203 LOI

0.43-8.7% (rising to 14.8% at the profile base) 2.3--7.6% 32--59% 5.2--35.6% 22.2--31.2%

Similar major-element concentrations and variations have been reported for bauxites in Suriname, Brazil, Venezuela and India {Harder, 1952; Balasubramanian, 1978; Mendoza et al., 1978; Aleva, 1979a, b). The distribution with profile depth of Si, A1 and Fe shows a linear relationship between Si and Fe and an inverse relationship between these elements and A1. The highest Si and Fe contents were found near the top and near the base of the profile. For Ti and Fe, a gradual decrease from the surface to

168

TABLE

I

C o n c e n t r a t i o n o f e l e m e n t s in t h e laterite soil profile Depth (ro)

% LOI

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.() 4.5 5.0 5.5 6.0 6.5 7.(t 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 ] 2.0 12.5

22 23 25 26 26 26 27 26 26 29 29 29 29 31 31 31 31 31 31 29 28 25 26 23 23

at 1050°C

X R F *L at 700°C

18

20

24

l4

AAS*:

Al (%)

Si (%)

Ti (ego)

Fe (q'v)

Sr (ppm)

Al (%)

Si (%)

Fe (%)

Na (ppm)

Mg (ppm)

Mn (ppm)

Ni (ppm)

Zn (pprn)

21 17 21 23 24 25 27 27 28 28 27 27 29 30 29 29 30 31 31 30 30 26 27 24 24

4.1 2.22 1.3 0.73 0.52 0.43 0.29 0.20 0.20 0.22 0.31 0.33 0.25 0.16 0.24 0.35 0.32 0.25 1.2 0.93 0.93 2.4 1.8 6.9 4.1

4.5 3.6 2.8 3.0 3.2 3.2 2.9 4.0 4.1 2.7 2.7 2.3 2.3 1.6 1.4 1.6 1.7 1.9 1.7 2.6 3.7 2.5 2.0 1.9 2.1

16 25 19 17 16 13 11 10 10 10 11 11 8.3 5.9 6.6 7.9 5.8 4.6 3.6 4.3 4.5 11 11 9.2 13

27

15

3.6

14

330

1,900

590

58

]80

23

0.33

10

230

1,000

350

53

14()

27

0.23

4.9

140

210

170

46

100

120 210 24

24 23 24

0,82 2.1 1.5

5.6 10 9.8

180 64 38

1,900 890 380

570 300 190

50 65 65

80 110 100

14

22

3.1

12

42

430

220

78

180

6

3

8

9

17

7

29 20 38 36 25 30 19 20 16

S.D. .4 BCR-I:

0.5

2

V a l u e s used in this w o r k

B X - N : V a l u e s used in this w o r k based o n N A A

of BCR-1

* I X-ray fluorescence. * 2 Atomic absorption spectrometry. * ~ N e u t r o n a c t i v a t i o n analysis. , 4 S t a n d a r d d e v i a t i o n , %.

a few meters above the base of the profile is seen. A significant enrichment in Ti is observed in samples at depths of ~ 4 - - 5 and 10--11 m. Trace e l e m e n t s

The trace elements investigated are strongly associated with the Fe fraction and only traces are present in the NaOH solution. The vertical distribution patterns for Fe, Co, Ti, Cr and Sc are given in Fig. 4 and for La, Ce, Sm and Eu in Fig. 5. Curves are smoothed according to the precision of the analytical methods. The distribution patterns of Co and of Ti resemble that of Fe203 (Fig. 2) with enrichments at ~ 4 and ~ 10--11 m, indicating isomorphous substitution o f Co and Ti for Fe in the hematite structure. A relatively high Cr content observed near the top and at 8-m depth may be associated with the

169

N AA ~ *

Depth

Fe (%)

Sc (ppm)

16 24 17 15 15 12 11 12 11 8.8 9.7 9.4 7.5 5.3 5.5 6.5 5.6 4.4 3.8 4.9 5.5 11 10 8.9 12

22 19 18 19 20 21 23 29 29 22 26 28 27 23 27 20 30 28 26 35 41 38 36 38 48

6 9.6 16

5

La (ppm)

40 39 48 48 47 71 89 66 76 34 33 28 26 17 15 16 3{~ 14 ]5 1 ",' 22 46 22 23 34 14

Ce (ppm)

Sm (ppm)

Eu (ppm)

Tb (ppm)

Tm (ppm)

73 70 93 89 87 130 150 120 140 66 65 50 48 37 31 32 68 32 27 43 36 95 36 37 61

5.2 4.2 6.9 4.4 4.3 6.2 7.4 7.6 7.8 5.7 5.2 4.7 5.3 3.7 2.1 2.5 4.0 2.1 2.0 2.5 5.0 18.0 3.5 3.3 5.2

1.4 1.3 1.5 1.6 1.1 1.3 1.5 1.6 1.8 1.3 1.2 1.2 1.4 0.9 0.6 0.6 0.8 0.6 0.6 0.8 1.5 7.6 1.3 1.2 1.4

] .1 0.8 0.9 0.8 0.6

0.4 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.~ 0.2 0.2

12

32

23

47

60

310

500

12 6.3 22

24 2.0 4.0

0.9 0.6 0.6 0.5

0.5 0 5

Hf (ppm)

Cr (ppm)

18 10 6

290 470 400 360 340 290 280 340 300 330 330 370 350 330 380 500 390 290 280 310 240 330 280 280 240

5 3

4 2 2

0.8

0.6

40

0.2 0.2 0.1 0.1 0.2 0.3 0.4 0.2 0.2 0.3 40

40

30 14.2 310

Co (ppm)

(m)

8.8 6.0 5.7 5.5 6.1 6.9 6.3 9.0 8.0 4.0 3.6 3.4 3.3 3.4 3.5 3.6 3.7 3.8 4.0 4.1 6.5 4.4 2.8 4.5 3.4

0.5 1.0 ] .5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6 0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5

21 38 36

relative enrichment of FeO(OH) (goethite). Sc seems to be the only element investigated which increases with depth. Compared to some karstic bauxite profiles, the variation ranges for La, Ce, Sm and Eu in the lateritic Suriname profile are narrow (Table II). However, the vertical distribution patterns for La, Ce, Sm and Eu (Fig. 5) are quite different from those obtained for the major elements. The lighter elements, La and Ce, are most strongly enriched at 2.5--5 m whereas the most pronounced enrichment for the intermediate ones, Sm and Eu, occurs near the base (11 m). No significant enrichment is obtained for heavier lanthanoids (Tb, Tm) or Hf. The strong mutual fractionation of the lanthanoids is further supported by results obtained using cluster analysis. Variables with highest mutual similarity (largest correlation coefficients, r) are clustered together as illustrated in the dendrogram of Fig. 6. Assuming a two-sided significance

170 level o f 1 0 % o r 1%, t h e c r i t i c a l v a l u e o f r is 0 . 3 3 o r 0 . 5 0 r e s p e c t i v e l y . T h u s a s t r o n g l i n e a r r e l a t i o n s h i p b e t w e e n Ce a n d L a (r = 0 . 9 9 ) a n d b e t w e e n E u a n d S m (r = 0 . 8 7 ) is o b t a i n e d w h e r e a s n o s i g n i f i c a n t r e l a t i o n s h i p (r = 0 . 2 2 ) is o b t a i n e d b e t w e e n t h e t w o p a i r s ( c o p h e n e t i c v a l u e 0 . 9 ) .

TABLE II Concentration of lanthanoids in laterites/bauxites Suriname profile

Element

La Ce Sm Eu

average

range

Bauxite standard* 1 BX-N

37 69 5.3 1.4

(14--89) (27--150) (2.0- 18) (0.6-.-7.6)

314 504 22.2 4.0

Karstic bauxites .2 average

range

306 393 106 25.8

(33--1,700) (55- 2,700) (13.9- 536) (3.5--120)

• 1 This work. • 2 Maksimovid and Roaldset (1976). 7

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Fig. 4. The vertical distribution of Fe, Co, Ti, Cr and Sc.

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I

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~

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/Depth(m) Fig. 5. The vertical distribution of La, Ce, Sm and Eu. Ce

La

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.&

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Fig. 6. Dendrogram for the mutual relationships between Ce, La, Co, Fe, Cr, Eu, Sm and Sc (based on Davis, 1973). The strong relationship between Ce and La, and Eu and Sm, respectively, and the poor correlation between the (La, Ce) and (Eu, Sin) groups may be noted.

172 The distribution pattern for Sr is very similar to that of Eu. It is difficult to explain this by ionic substitution as the ionic radii for Sr ~÷ (1.12 A) differs from that of Eu 3÷ (0.95 A), which is the prevailing state of Eu. A common transport mechanism does, however, seem probable. Similar fractionation among light, intermediate and heavy lanthanoids has been reported by Ronov et al. (1967) who assign this phenomenon to pH variations with depth. However, as illustrated in Fig. 6, La and Ce are relatively enriched at three levels, with maximum concentrations between 2.5 and 5 m in depth. The level near the base (11 m) corresponds to the enrichment level of Sm and Eu. These maxima (2.5 5- and l l - m depths) correspond to zones of high hematite (Fe203) content (Fig. 2). Furthermore, goethite and limonite could not be detected by XRD at the above-mentioned levels, although observed in the other parts of the profile. Therefore, an incorporation of lanthanoids into the hematite structure during the growth of this secondary mineral is assumed. All primary minerals have been dissolved during the weathering process. A variable distribution of lanthanoids, therefore, must be due to differences in mobility and capacity for incorporation into the structure of secondary minerals. As no enrichment for Eu and only a slight enrichment for Sm are observed in the upper hematite zones, a higher mobility of these elements compared to that of La and Ce is indicated. Furthermore, the absence of maxima for heavier lanthanoids within the profile depth investigated may indicate removal, i.e., an even higher mobility for these elements. The present data show that the different lanthanoids are concentrated at various levels within the weathering profile and therefore indicate that they are strongly fractionated by weathering processes. CONCLUSION In the investigated laterite profile, gibbsite is the dominant mineral, constituting up to 90% in the intermediate part, whereas goethite and limonite a m o u n t to 3 0 - 4 0 % in the profile cap. Kaolinite appears downward, and increases towards the profile base. Other minerals present are hematite and quartz. The profile represents a tropical weathering profile in the lateritic stage {Kuzvart, 1978). The major-element composition and variation with depth reflect the mineralogy, with strong accumulation of A1203 in the intermediate part and some relative enrichment of Fe203 and SiO2 towards the top and the base of the profile. The extraction experiments show that all trace elements investigated are mainly associated with the Fe-rich phase (red mud). The trace-element distribution within the investigated lateritic weathering crust is complex. The results clearly show that the trace elements are differentiated with depth and have maxima at various levels. The most surprising fractionation is, however, found among the light and the intermediate

173 l a n t h a n o i d s ( L a , Ce, S m , E u ) , b e i n g s t r o n g l y s u p p o r t e d b y t h e c l u s t e r a n a l y s i s . T h u s e v e n s m a l l d i f f e r e n c e s in t h e i n n e r e l e c t r o n levels o f t h e lanthanoid elements apparently influence their mobility and fractionation during intense weathering processes. ACKNOWLEDGEMENTS Samples were provided by Viggo Wiik, ~rdal & Sunndal Verk. The authors wish to thank Professor A.C. Pappas, Department of Chemistry, University of Oslo, for valuable discussions and for having read through t h e m a n u s c r i p t . B. S a l b u e x p r e s s e s h e r a p p r e c i a t i o n t o t h e N o r w e g i a n M i n i s t r y o f C u l t u r e a n d S c i e n c e f o r f i n a n c i a l s u p p o r t . T h e a u t h o r s a r e indebted to the Royal Norwegian Research Council for Scientific Technology for financial support given to this project.

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174 Harder, E.C., 1952. Examples of bauxite deposits illustrating variation in origin Problems of clay and lateritic genesis. Am. Inst. Min. Metall., pp. 35--64. Keller, W.D., 1957. The Principles of Chemical Weathering. Lucas, Columbia, Mo. Krook, L., 1975. Surinam. In: R.W. Fairbridge (Editor), The Enclopaedia of World Regional Geology, Part 1. Western Hemisphere (Including Antarctica and Australia). Dowden, Hutchinson & Ross, Stroudsburg, Pa., pp. 480--492. Kuzvart, M., 1978. Kaolin genesis. Episodes, 4: 12--15. Lelong, F., 1969. Nature et gendse des produits d'alt~ration de roches cristallines sous climat tropical humide (Guyana Fran~aise). Sci. Terre Mere., 14. Loughnan, F.C., 1969. Chemical Weathering of Silicate Minerals. Elsevier, Amsterdam. Maksimovi5, Z., 1976. Genesis of some Mediterranean karstic bauxite deposits. Trav. Int. Congr. for Study of Bauxites, Alumina and Aluminium (I.C.S.O.B.A.), 13: 1- 14. Maksimovid, Z., 1978. Geochemical study of the Marmara Bauxite deposit: Implication for the genesis of Brindleyite. Trav. Int. Congr. for Study of Bauxites, Alumina and Aluminium (I.C.S.O.B.A.), No. 15. Maksimovid, Z. and Roaldset, E., 1976. Lanthanide elements in some Mediterranean karstic bauxite deposits. Trav. Int. Congr. for Study of Bauxites, Alumina and Aluminium (I.C.S.O.B.A.), 13: 199- 220. Mendoza, V., Sifontes, R.S. and Rodriguez, S.E., 1978. The Pijignaos bauxite deposits, western Bolivar, Venezuela. 4th Int. Congr. for Study of Bauxite, Alumina and Alumin i u m ( I . C . S . O . B . A . ) , 2 : 5 8 5 586. Ronov, A.B., Balashov, Y.A. and Migdisov, A.A., 1967. Geochemistry of the rare earths in the sedimentary cycle. Geochem, Int., 1 4 : 1 17. Sahasrabudhe, Y.S., 1978. Geochemistry of bauxite profiles on different rock types from central and western India. 4th Int. Congr. for Study of Bauxite, Alumina and Aluminium (I.C.S.O.B.A.), 2: 734--751. Salbu, B., Steinnes, E. and Pappas, A.C., 1975. Multielement neutron activation analysis of fresh water using Ge(Li) gamma spectrometry. Anal. Chem. 47(7): 1011 1016.