Geochemical Journal, Vol. 35, pp. 13 to 28, 2001

13

Mineralogical and chemical composition and distribution of rareearth elements in clay-rich sediments from central Uganda

GEORGE W. A. NYAKAIRU and CHRISTIAN KOEBERL*

Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

(Received August 11, 2000; Accepted October 28, 2000)

Clay-rich sediments from the Kajjansi, Kitiko, Kitetika, and Ntawo valleys in central Uganda wereanalyzed for mineralogical and chemical composition, including the rare earth element (REE) contents.The valleys are filled with Quaternary to Recent alluvial and lacustrine sands, silt, and gravels, whichformed from the bed rock metasediments of the Buganda-Toro System and from granitoid rocks that in-clude rocks of the basement. The sediments are dominated by kaolinite and quartz, and minor phasesinclude smectite, chlorite, and illite/muscovite. Whole rock chemistry shows that sediment samples rich inSiO2 have low Al, Fe, Sc and Cr contents. The high chemical index of alteration (CIA) values (87 to 96),chemical index of weathering (CIW) values around 98 and low contents of the alkali and alkali earthelements of the clay-rich sediments suggest a relatively more intense weathering source area. Barium, Rb,Ca, and Mg were probably flushed out by water during sedimentation. The chondrite-normalized REEpatterns of the clay-rich sediments show LREE enrichments and a negative Eu anomaly. The high chondrite-normalized La/Yb ratios, and Gd/Yb ratios lower than 2.0, indicate that the sediments are enriched in theLREEs and are similar to typical post-Archean shales, such as Post-Archean Australian Shale (PAAS).The mineralogical composition, REE contents, and elemental ratios in these sediments suggest a prov-enance from mainly felsic rocks, with only minor contributions from basic sources. The basic sedimentswere most likely derived from metasedimentary rocks, such as muscovite-biotite schists, which are char-acteristic of the Buganda-Toro System rocks, whereas the felsic sediments are derivatives of granitoidrocks of the basement. The most significant geochemical finding is that despite intense weathering, whichhas affected most elements, the REE, Th, and Sc remain immobile.

ten contains boulders of laterite, which may be upto 1.5 m in diameter (Tuhumwire et al., 1995). Itis variable in distribution and thickness and usu-ally covered by soils. The valleys are filled withpapyrus swamps and drain towards Lake Victo-ria. The sedimentary rocks exposed in these val-leys are underlain by granitoids from the basementand metasediments of the Buganda-Toro System(Fig. 1(b)). The basement is composed mostly ofigneous and metamorphic rocks of uncertainPrecambrian age, with some outcrops ofPaleozoic, Mesozoic, or Cenozoic rocks (Holmes,1951). The Buganda-Toro system extensivelyoverlies the basement. It occupies much of thesouth, west, and central parts of Uganda and

INTRODUCTION

A suite of sediments from central Uganda wascollected to study their mineral and chemical com-position, including rare earth element (REE) pat-terns, and to determine their source rocks. Thesamples were taken from the quarries at Kajjansi,Kitiko, Kitetika, and Ntawo, which are located inbroad valleys with gentle gradients cut in hillscapped with laterite. The flat-topped lateriteduricrust capped hills form noteworthy topo-graphic features with murram and boulders occur-ring on hilltops. Murram is essentially a loosegravel made up of fragments of concretionarylaterite up to 2.5 cm or 5 cm in diameter, but of-

*Corresponding author (e-mail: christian.koeberl@univie.ac.at)

14 G. W. A. Nyakairu and C. Koeberl

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Clay-rich sediments from central Uganda 15

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16 G. W. A. Nyakairu and C. Koeberl

stretches from Jinja to the Rwenzori mountains inwestern Uganda (Fig. 1(a)). The Buganda-Tororocks are of low metamorphic grade and comprisemainly arenites, argillites, phyllites, schists, grits,granites, and meta-volcanics (McConnell, 1959).

In this study, the sediments sampled were de-rived from granitoids and other Precambrian rocksof the basement and metamorphosed Buganda-Toro System. These sediments are heavily altered,mostly to clays (McGill, 1965). Locally, theseclay-rich sediments have been used to producebricks and pottery. All previous studies of thesesediments have been directed towards these ap-plications (e.g., McGill, 1965; Nyakairu andKaahwa, 1998; and references therein). Despitethese studies, the chemical and mineralogical com-positions of these sediments, and their sources,are still poorly understood. The main aim of thisstudy is to determine the mineralogical andgeochemical composition of these sediments, withspecial focus to the behavior of the REEs. Theresults are discussed in terms of source area weath-ering and provenance.

DESCRIPTION OF STUDY AREAS

KajjansiKajjansi valley is a north-western extension of

the Murchison bay (area 1 in Fig. 1(b)). The val-ley is filled with Quaternary to Recent alluvial andlacustrine sands, silt, and gravels, which formedfrom metasediments of the Buganda-Toro Systemand from basement granitoids (Fig. 1(b)). Sam-pling was carried out at the main pit of UgandaClays Limited factory. The pit has a depth of about3 to 4 m, which, in places, extends to 5 m. Thedeposit can be divided into three layers: a graylayer overlying a dark gray or near black layer,which, in turn, overlies a light gray layer. The graylayer thickness varies from 60 to 100 cm, a darkgray layer varies from 90 to 160 cm and a lightgray layer varies from 60 cm onwards untilsaprolite of the bedrock is reached. In some places,the dark gray layer exhibits what appears to bebedding surfaces with impressions of plant leavesand stems, as well as thin laminations of silt and

some mud balls. Decomposed swamp vegetationis abundant in both the gray and dark gray layers.The overburden encountered was a thin layer offine white sand or a black clayey soil with an av-erage thickness of 30 cm.

KitikoThe outcrops are sediments from a former lake,

which was part of Lake Victoria during climatic

Fig. 2. Field photograph taken at Kitiko pit showingsampling method used. In the photograph two layersare identified. The uppermost dark gray layer that isrich in organic matter is underlain by the gray to brownlayer, which becomes more brown near the top of thewater table. In the top left corner is a pile of locallymade bricks and onlookers are in the right hand cor-ner. The sample was collected and placed in the sam-ple container at the very location, and a measuring rodwas used to determine the pit depth. The samplingmethod used was similar in all the four sediment out-crops.

Clay-rich sediments from central Uganda 17

changes in the Pleistocene and Holocene (Bishop,1969). The surface cover comprises cemented sandand gravel, which is up to a meter thick. How-ever, in some parts of the pit, the surface cover isrepresented by a near black layer rich in organicmatter as shown in photograph (Fig. 2). Below thisis a 3–5 m thick sediment layer, which is locallyused in brick and tile making. No bedding due tospecific grain-sizes was observed. The sedimentis gray to brownish, becoming more brown nearthe water table (Fig. 2).

KitetikaThe valley is filled with Quaternary to Recent

alluvial and lacustrine sands, silt, and gravels,

which are derived from underlying metasedimentsof Buganda-Toro system and granitoid rocks ofthe basement (Fig. 1(b)). The outcrop is 1 to 2 mthick, and is composed of a sandy sediment, withbedding of different colors. In a pit dug in thisarea, three distinct horizontally bedded layers,whose thicknesses are quite different from eachother were observed. The bottom layer was sam-pled, which comprises a brownish-gray sediment,with small sandstones, a middle, grayish and anuppermost black layer due to organic matter.

NtawoThe Ntawo area comprises horizontally layered

sediments, with a thickness of approximately 2 to

Table 1. Semi-quantitative (wt%) mineralogical composition of Kajjansi, Kitiko,Kitetika, and Ntawo sediments, Central Uganda, analysed by X-ray diffractometry.Calculations performed using a modified method after Schultz (1964).

Sample Depth (cm) Kaolinite Chlorite Smectite Quartz Feldspars Calcite

KAJJANSI DEPOSITKJ-1 390 27 11 3 12 48KJ-2 300 45 9 7 38 1KJ-3 240 39 19 15 25 3KJ-4 120 46 14 19 18 4KJ-5 60 34 64 1KJ-6 30 26 8 19 45 3

KITIKO DEPOSITKS-1 240 67 15 16 2KS-2 210 75 24 1KS-3 180 54 32 14KS-4 150 34 12 19 31 4KS-5 120 62 38KS-6 90 55 23 21 1KS-7 60 54 28 18KS-8 30 44 55 1

KITETIKA DEPOSITKT-L 120 51 26 23KT-M 90 36 62 2KT-U 60 30 67 3KT-A 30 18 4 78

NTAWO DEPOSITNT-1 180 32 16 49 3NT-2 150 47 46 7NT-3 120 19 77 4NT-4 90 22 8 64 7NT-5 60 30 7 10 50 3NT-6 30 24 13 20 39 3 1

18 G. W. A. Nyakairu and C. Koeberl

3 m, which developed from the granitoid basementrocks. The bottom layer consists of a gray/brownsediment, which is overlain by a gray layer, fol-lowed by a gray sandy layer and a brown/dark soilcover with abundant organic matter. A pit dug inthe center of the Ntawo valley shows three dis-tinct layers. A gray layer at the bottom is overlainby a black layer together make a sediment that isabout 2 m thick. This sediment is overlain by 0.5to 1 m of soil cover.

MATERIALS AND METHODS

For this study, we analyzed 24 sediment sam-ples from Kajjansi, KJ (n = 6); Kitiko, KS (n = 8);Kitetika, KT (n = 4), and Ntawo, NT (n = 6). Rep-resentative samples were collected at differentdepth (Table 1) from the bottom to the top of eachof the outcrops, corresponding, to macroscopicfeatures (e.g., color of different layers) (Fig. 2).Sample masses were reduced to 250 g by coningand quartering, followed by drying at 60°C for 12h. Aliquots of 20–30 g of each dried sample werepowdered in an agate mill. Mineralogical analy-sis was performed using a Philips powder X-raydiffractometer (PW 3710), operated at 45 kV/35mA using Ni-filtered CuKα radiation, with auto-matic slit and on-line computer control. Semi-quantitative analyses were made using a modifiedmethod after Schultz (1964). The samples wereX-rayed using both random powder and orientedslides for identification of mineral composition,and the diffractograms were processed usingPhilips PC-APD software, version 3.6.

Major element oxides and some trace elements(Sr, Y, Zr, Nb, Ni, Cu, V, and Ba) concentrationswere determined by X-ray fluorescence (XRF)spectrometry at the University of theWitwatersrand, Johannesburg, South Africa. De-tails of the XRF method are described by Reimoldet al. (1994). Other trace elements were deter-mined at the Institute of Geochemistry, Univer-sity of Vienna (Austria), by instrumental neutronactivation analysis (INAA). Samples weighingabout 150 mg each were sealed in polyethylenecapsules and, together with rock standards AC-E

and Allende, G-2 (cf., Govindaraju, 1989;Bowman, 1994), irradiated for 8 hours in the 250kW Triga reactor of the Atominstitut derÖsterreichischen Universitäten, Vienna (Austria),at a neutron flux of 2.1012 n cm–2s–1. For detailsof the INAA method, including precision and ac-curacy, see Koeberl (1993).

MINERALOGY

The XRD patterns of all whole-rock samples,which were obtained following the method ofBrown and Brindley (1984), indicate the predomi-nating presence of kaolinite and quartz, with mi-nor muscovite or illite. Iron mineral peaks werenot observed, indicating that no hematite was in-corporated in the clay minerals. Smectite andchlorite are present in trace amounts in some ofthe samples. Identification of secondary mineralswas difficult, because their peaks tended to beobscured by the greater peaks of the major miner-als kaolinite and quartz. Semi-quantitative analy-ses, following a method modified after Schultz(1964), showed differences in bulk mineral com-positions between the individual sediments (Ta-ble 1). For example, the kaolinite content variesfrom 18 wt% for KT-4 to 67 wt% for KS-1, withsamples much richer in quartz being poorer inkaolinite. The KS samples contain high amountsof kaolinite, with two of the samples consistingof smectite. The NT and KT samples have highquartz contents, with a few of them showing mi-nor amounts of chlorite and smectite. All of thesediment samples contain only small amounts offeldspar and calcite, except for KJ-1, which has48 wt% feldspar. All sediments are, therefore, richin kaolinite and quartz, which indicate that theyare derived mainly from felsic sources. The mi-nor illite and illite-weathered products, such assmectite and chlorite present in some samples, andthe low feldspar abundance, suggest a contribu-tion from basic sources. The felsic sediments arederivatives of granitoid basement rocks, whereasthe basic component is derived from the basicschists of either the basement or the Buganda Se-ries (cf., Cullers, 1994a).

Clay-rich sediments from central Uganda 19

WHOLE-ROCK GEOCHEMISTRY

Table 2 lists the results of the major and traceelement analyses for the Ugandan sediments com-pared to the average compositions of the Singogranite from Nagudi et al. (2000); the upper crust(UC) and Post-Archean Australian Shale (PAAS),from Taylor and McLennan (1985), and the NorthAmerican Shale Composite (NASC), from Grometet al. (1984). The Singo granite occurs in westerncentral Uganda, intrudes into local metasediments,and is one of the few granitoids in Uganda thathas been studied for its trace element composi-tion. Thus, it can be used as a proxy for granitesin the study area. As expected from weatheredrocks, there is a large variation in the major ele-ment content for all analyzed samples. For exam-ple, the SiO2 contents range from 51 to 85 wt%,the TiO2 content from 0.58 to 1.67 wt%, the Al2O3

content from 7.61 to 27.08 wt%, and the Fe2O3

content ranges from 1.46 to 12.20 wt%. The con-tents of MgO, CaO, and MnO are all low. TheNa2O and K2O contents range from 0.04 to 0.17wt%, and from 0.56 to 1.09 wt% respectively.Nesbitt and Young (1982, 1984, 1989, 1996) and

Nesbitt et al. (1996) used the ternary diagramsAl2O3-(CaO + Na2O)-K2O (the A-CN-K), diagramand Fe2O3 + MgO-(CaO + Na2O + K2O)-Al2O3

(the A-CNK-FM), diagram to deduce weatheringtrends. On both the A-CN-K diagram (Fig. 3), andthe A-CNK-FM diagram (Fig. 4), all the sedimentsdisplay an intense weathering history. Thesediments plot in a region clearly suggesting dif-ferent relative contents in Al2O3, CaO, Na2O, andK2O from those of Singo granite, PAAS, NASC,and UC, and plot closer to high Al2O3 contents,which is suggestive of a relatively high intensityof weathering. Weathering has proceeded to astage at which significant amounts of the alkaliand alkali earth elements were removed from thesediments. Albite has not been identified in thesamples, indicating that the sediments are mature.

Weathering studies show that Ca, Na, and Srare rapidly lost during chemical weathering andthe amount of these elements lost is proportionalto the degree of weathering (Wronkiewicz andCondie, 1987, and references therein). Two chemi-cal weathering indices have been proposed: CIA(chemical index of alteration); Nesbitt and Young(1982), and CIW (chemical index of weathering);

Fig. 3. Al2O3-(CaO + Na2O)-K2O plot of sediment sam-ples (after Nesbitt and Young, 1982, 1984, 1989, 1996;Nesbitt et al., 1996), compared to data for Post-ArcheanAverage Shale (PAAS) and Upper Crust (UC) given byTaylor and McLennan (1985); and North AmericanShale Composite (NASC) given by Gromet et al. (1984).

Fig. 4. Triangular Al2O3-(CaO + Na2O + K2O)-Fe2O3 + MgO plot of sediment samples (after Nesbittand Young, 1982, 1984, 1989, 1996; Nesbitt et al., 1996)in comparison with Post-Archean Average Shale andUpper Crust (data from Taylor and McLennan, 1985)and North American Shale Composite (data fromGromet et al., 1984).

20 G. W. A. Nyakairu and C. Koeberl

Table 2. Major and trace element composition of Kajjansi, Kitiko, Kitetika, and Ntawo sediments, Central Uganda

Harnois (1988). Nesbitt and Young (1982) re-ported a CIA value of nearly 100 for kaolinite andchlorite, and 70 to 75 for average shales, whereasTaylor and McLennan (1985) reported a CIA valueof 85 to 100 for residual clays. Condie (1993) re-ported that most post-Archean shales show mod-erate losses of Ca, Na, and Sr from source weath-ering, with CIW values of 80 to 95 and Sr con-

tents of 75 to 200 ppm. In contrast, most Archeanshales, show greater losses of all three elements,with CIW of 90 to 98, and Sr contents < 100 ppm.The CIA of the sediments varies between 87 to96, which is high compared to PAAS, UC, andNASC, but close to that of kaolinite (Table 2). TheCIW, which does not include K2O, ranges between96 to 99, with Sr contents ranging from 30 to 61

Clay-rich sediments from central Uganda 21

Major elements as wt% and trace elements in ppm. Total Fe as Fe2O3, CIA = [Al2O3/(Al2O3 + CaO + Na2O + K2O)] × 100 andCIW = [Al2O3/(Al2O3 + CaO + Na2O)] × 100 in molecular proportions. CaO in both CIA and CIW is CaO in silicate fractiononly and was corrected following the McLennan (1993) method. SINGO = Average Singo granite (data from Nagudi et al.,2000); PAAS = Average Post-Archaean Australian Shale; UC = Upper Crust (data from Taylor and McLennan, 1985) and

NASC = Average North American Shale (data from Gromet et al., 1984). Eu/Eu* = Eucn/ [(Sm )(Gd )]cn cn .

Table 2. (continued)

22 G. W. A. Nyakairu and C. Koeberl

ppm. The CIA and the CIW are both higher thanthose of average shales, suggesting relatively in-tense source area weathering. The high CIA andCIW values in the sediments probably reflect thepresence of clay minerals and absence of detritalfeldspars.

TRACE ELEMENT GEOCHEMISTRY

The behavior of trace elements during sedi-mentary processes is complex due to factors in-cluding weathering, physical sorting, adsorption,provenance, diagenesis, and metamorphism (e.g.,Garrels and Mackenzie, 1971; Krönberg et al.,1979; Nesbitt et al., 1980; Taylor and McLennan,1985; Wronkiewicz and Condie, 1987). The traceelement data show large variations compared toPAAS, NASC, and UC (Table 2). All sedimentshave similar contents in trace elements and areenriched in Y, Zr, Nb, Hf, Ta, Th, and U. Theseelements are associated with heavy minerals, suchas zircon, which is resistant to weathering. Theyare preferentially partitioned into melts duringcrystallization and anatexis (Feng and Kerrich,1990), and as a result, these elements are enrichedin felsic rather than mafic rocks. Thorium and Ubehave differently during weathering, as U, un-like Th, is chemically mobile as U6+ and there is atendency of the Th/U ratio to be elevated aboveupper crustal values of 3.5 to 4.0 (McLennan etal., 1993). The sediments show Sc, V, Zn, Ni, andCr, contents similar to, or even above, abundancesfor PAAS. However, they are depleted in Co, Cu,Rb, Sr, as well as in Cs, and Ba contents. Gener-ally these elements are associated with clay min-erals, and their low values could be attributed tothe low values in parent rocks and the high quartzcontent in the sediments (Table 1). This is in agree-ment with studies by Nesbitt et al. (1980) andWronkiewicz and Condie (1987), in which theseauthors conclude that small cations, such as Na,Ca, and Sr, are selectively leached and weatheredfrom weathering profiles, whereas cations withrelatively large ion radii, such as K, Cs, Rb, andBa, may be fixed by preferential exchange andadsorption on clays. Therefore, in general, weath-

ering conditions were more intense in the sourcearea before sedimentation, compared to the con-ditions for PAAS.

Strontium and barium mostly reside inplagioclase and K-feldspar, respectively (Puchelt,1972). The fractionation of these two elements canresult from the selective weathering of thesefeldspars. Weathering of plagioclase will decreasethe Sr content, because plagioclase is more easilyweathered than K-feldspar, and Sr is more mobilethan Ba. A depletion of Ba could be due torecrystallization of clays and progressive destruc-tion of feldspars. Potassium, Rb, and Cs are mainlyhosted in micas and K-feldspar (Heier andBillings, 1970); thus, alteration of these mineralswill dominate the fractionation of these elements.Rubidium has a trend comparable to that of Nband V and is less mobile than K, whose behaviorit otherwise follows. Rubidium with respect to Kis preferentially retained in weathered illite(Garrels and Christ, 1965). Niobium and V are lessmobilized, in agreement with what is known abouttheir behavior during weathering (cf., Middleburget al., 1988). The elements As and Sb are concen-trated in samples that are enriched in iron oxides(Fe2O3) due to weathering under arid oxidizingconditions.

RARE-EARTH ELEMENT VARIATIONS

The results of the REE analyses are summa-rized in Fig. 5. All analyzed sediment sampleshave similar concentrations of the REEs.Chondrite-normalized patterns are typical forshale in general, with an enrichment of the LREEs.All samples have pronounced negative Eu anoma-lies, ranging from 0.57 to 0.90. The Eu anomalyparallels the depletion in Na2O and CaO, suggest-ing that it developed at least partially in responseto plagioclase weathering, where most of the Euis hosted. The Eu anomaly in sedimentary rocksis usually interpreted as being inherited from ig-neous source rocks (McLennan and Taylor, 1991;Taylor and McLennan, 1985; Awwiller, 1994). Fig-ure 5 illustrates that despite the difference in theabsolute abundances, the samples show REE pat-

Clay-rich sediments from central Uganda 23

terns that are similar to those of NASC and PAAS,but differ in the HREE content compared to theUC. The chondrite-normalized La/Yb ratio rangesfrom 8.3 to 14.5, with most sediments havingslightly higher values than those of PAAS and UC.The relative depletion in the HREEs compared toLREEs may be due to a lower concentration ofheavy minerals, for example zircon. The low abun-dances of Zr in the sediments (Table 2) supportsthis interpretation.

PROVENANCE

The REEs and high field strength elements(HFSE, including Y, Zr, Ti, Nb, Ta), Th, Sc, Hf,and Co are the most suitable ones for provenancedetermination, because of their relatively lowmobility during weathering, transport, diagenesis,and metamorphism. Ratios of both incompatibleand compatible elements are useful for differenti-ating between felsic and mafic source components.

In addition, the REE patterns have been also usedto infer sources of sedimentary rocks, since basicrocks contain low LREE/HREE ratios and no Euanomalies, whereas more silicic rocks usuallycontain higher LREE/HREE ratios and negativeEu anomalies (e.g., Cullers and Graf, 1983).Therefore, the REE patterns of the source rocksmay be preserved in sedimentary rocks (Taylor andMcLennan, 1985; Wronkiewicz and Condie, 1987,1989).

Mineralogical maturity of the sediments indi-cated by the proportion of compositionally ma-ture alumina-rich minerals such as kaolinite isconsistent with having a felsic or reworked sedi-mentary source (e.g., Cullers et al., 1975, 1987).The CIA, which varies from 87 to 96, and the CIWvalues (96 to 99) point towards a more felsicsource and may indicate sediment recycling proc-esses (Nesbitt et al., 1980). The sediments havefractionated REE patterns, with (La/Yb)cn vary-ing from 8.3 to 14.5, and a negative Eu anomaly

Fig. 5. Rare earth element plots of sediment samples normalized to C1 chondrites after Taylor and McLennan(1985). (a) Kajjansi, (b) Kitetika, (c) Kitiko and (d) Ntawo samples.

24 G. W. A. Nyakairu and C. Koeberl

(0.57 to 0.90). The LREEs are enriched and theHREE patterns are almost flat, with (Gd/Yb)cn

varying from 0.81 to 1.36. These characteristicsindicate that the original source area was felsicand the negative Eu anomaly is regarded as evi-dence for a differentiated source, similar to gran-ite (McLennan, 1989; McLennan et al., 1993;Taylor and McLennan, 1985, 1995). The relativeenrichments of (normally) incompatible elements(e.g., LREE, Th) over compatible elements (e.g.,Sc, Co) in the sediments, indicate relatively felsicaverage provenance compositions and a relativelysevere weathering regime (McLennan et al.,1993). The slight differences among the REE pat-terns probably do not reflect changes in sourcearea composition, but are likely due to variationsin mineral sorting and weathering.

The immobile elements La and Th are moreabundant in felsic than in basic rocks, whereas Scand Co are more concentrated in basic rocks thanin felsic rocks (Taylor and McLennan, 1985;Wronkiewicz and Condie, 1987). It has beenshown that ratios, such as La/Sc, Th/Sc, Co/Th,Cr/Th, and Eu/Eu*, in siliciclastic sediments, al-low to place constraints on the average provenancecomposition (e.g., Cullers et al., 1988; Cullers,1994b, 1995; Wronkiewicz and Condie, 1987,1989, 1990; Cox et al., 1995). Thus, the concen-

tration of these elements and the correspondingelemental ratios in sediments may be useful forprovenance determination. The geochemical dif-ferences between elements such as Th and La (in-dicative of a felsic source) and Sc and Cr (indica-tive of a mafic source) have been exploited to dis-tinguish between felsic and mafic provenance byvarious authors (e.g., McLennan, 1989; McLennanand Taylor, 1991; McLennan et al . , 1980;Wronkiewicz and Condie, 1990). The Th versusSc plot (Fig. 6(a)) shows that the sediment datascatter above the Th/Sc = 1 line with most sam-ples that have Sc contents below 15 ppm indicat-ing a more felsic component. The Th/Sc valuesare higher than that of UC. On the La/Th plot (Fig.6(b)), the sediment data fall into the range of UC,with most samples having a significant felsic com-ponent. This indicates that the sediments did nothave a uniform provenance. The ratios of Th/Scrange from 1.1 to 2.84 and La/Th from 1.87 to3.35, also suggesting a mixed source of sediments.

In a La-Th-Sc diagram (Fig. 7), which is usedto discriminate felsic and basic provenance of fine-grained sediments (e.g., Cullers, 1994a, b), thesediment data fall in a region that indicates a pre-dominantly felsic source, but does not exclude anintermediate source or possible mixing betweenfelsic and basic source rocks. The clustering of

Fig. 6. (a) Th versus Sc for sediment samples. Th/Sc = 1 ratio is that of the upper continental crust (UC) (b) Laversus Th for the sediment samples. The La/Th = 2.8 ratio is that of upper continental crust (data from Taylor andMcLennan, 1985).

Clay-rich sediments from central Uganda 25

sediment data more or less around the Singo gran-ite value suggest that the sediments are mainlyderived from a source with a composition similarto that of the Singo granite. The sediment datashow a cluster along the La-Th edge, at a positioncloser to La than to Th. The fact that all the sedi-ment data plot close to the values for UC, NASC,and PAAS indicates that, despite the intenseweathering experienced by the sediments, La, Th,and Sc have remained immobile. The sedimentshave low contents of Sc, Fe, and Cr (and Co), highconcentrations of the REE, Th, Hf, high La/Sc,Th/Sc, La/Cr, Th/Cr, and Ba/Sc ratios, which in-dicates that they were derived mainly from felsicrocks (cf., Cullers et al., 1987, 1988; Cullers,1988). It can be concluded that the sediments werederived from predominantly felsic sources, ratherthan from basic rocks. The sediments with a basiccomponent were most l ikely derived frommetasedimentary rocks, such as muscovite-biotiteschists, which are characteristic of the Buganda-Toro System rocks, while the felsic sediments arederivatives of granitoid rocks of the basement.Thus, on the basis of chemical composition, thesediments are derived locally, mainly from highlyacidic rocks, with minor contributions from basicrocks.

SUMMARY

We analyzed 24 sediment samples fromKajjansi, Kitiko, Kitetika, and Ntawo, centralUganda, in order to determine their mineralogicaland chemical compositions, REE characteristics,and their provenance. XRD analyses show that thesediments are dominated by kaolinite and quartz,with minor smectite, chlorite, and illite/muscovite.From the weathering diagrams (Figs. 3 and 4), itis possible to infer that the samples underwent arelatively high degree of weathering. Weatheringhas proceeded to a stage where a major part of thealkali and alkali earth elements were removed. Thelow CaO contents in sediments indicate their ma-turity relative to most typical post-Archean shales,such as PAAS and NASC. The sediments are de-pleted in Ca, Na, and Sr compared to Archeanshale compositions (Condie, 1993). CIW valuesof around 98, and variations in CIA values from87 to 96, indicate that the sediments contain re-sidual clays rich in kaolinite (Taylor andMcLennan, 1985).

The low contents of Sc, Fe, and Cr (and Co),and high abundances of the REE, Th, Hf, highLa/Sc, Th/Sc, La/Cr, Th/Cr, and Ba/Sc ratios in

Fig. 7. Ternary plot of La-Th-Sc for the sediment samples after Cullers (1994a) compared with Singo granite(data after Nagudi et al., 2000); Post-Archean Average Shale, Upper Crust (data from Taylor and McLennan,1985), and North American Shale Composite (data from Gromet et al., 1984).

26 G. W. A. Nyakairu and C. Koeberl

the sediments, all indicate that the sediments werederived from granitic rocks. This also agrees withthe general shape of the chondrite-normalizedREE patterns (including a negative Eu anomaly),which suggests that the granites—the likely sourcerocks—were derived by inter-crustal melting. Theslight increase in the LREE is probably a sourcearea effect. Mineralogical and geochemical datahere reported suggest that the provenance of thesediments was dominated by felsic sources. Thebasic component in the sediments was most likelyderived from metasedimentary rocks, such asmuscovite-biotite schists, which are characteris-tic of the Buganda-Toro System rocks, while thefelsic sediments are derivatives of granitoid rocksof the basement. We find that, despite intenseweathering, which affected the contents of mostelements, the REE, Th, and Sc remained immo-bile; this observation can be considered as one ofthe most important geochemical results of thepresent study.

Acknowledgments—We are grateful to W. U. Reimold(Univ. Witwatersrand, Johannesburg) for XRF analy-sis and S. Gier (Inst. of Petrology, Vienna) for assist-ance with XRD analysis. A Ph.D. scholarship from theAustrian Academic Exchange (ÖAD) to the first au-thor and financial assistance for fieldwork is gratefullyacknowledged. Laboratory work was supported by theAustrian FWF, project Y58-GEO (to C.K.). We aregrateful to S. R. Taylor (Australian National Univer-sity) and to an anonymous colleague for helpful andconstructive reviews, and to J. Matsuda for editorialcomments, all of which led to improvements in themanuscript.

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