Geochemical Journal, Vol. 35, pp. 245 to 256, 2001

245

NOTE

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

The Buwambo kaolin deposit in central Uganda:Mineralogical and chemical composition

GEORGE W. A. NYAKAIRU,1 CHRISTIAN KOEBERL1* and HANS KURZWEIL2

1Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria2Institute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

(Received February 12, 2001; Accepted June 20, 2001)

Kaolin from the Buwambo deposit, located about 27 km north of Kampala (Uganda), has been analyzedfor its mineralogical and chemical composition. The kaolin is derived from granite of the basement, whichis exposed due to deeply weathered Buganda-Toro cover rocks. Kaolinite is the dominant mineral, withquartz and muscovite/illite as accessory minerals. Chemical data show that the kaolin is composed mainlyof SiO2 and Al2O3, with the other oxides being present in trace amounts. The depletion in Ti, Fe, Mn, Mg,Ca, Na, and K not only shows the extent of the kaolinization, but also gives Buwambo kaolin its almostwhite color. The kaolinization and weathering processes have enriched Ni and depleted other trace ele-ment contents in the Buwambo kaolin. The chondrite-normalized rare earth element (REE) patterns showenrichment in the light REEs, with a negative Ce anomaly. The REE pattern and the content of the othertrace elements, show evidence of alteration and weathering processes related to kaolinization. The minera-logical and chemical compositions indicate that the kaolin is suitable for industrial use.

its occur, their industrial use is restricted to manu-facturing of bricks and pottery (Nyakairu andKaahwa, 1998, and references therein). However,small quantities of kaolin are locally used as whitewash or “enoni” on the mud and wattle dwellings.Apart from reconnaissance work by Harris (1946)and Pekkala and Katto (1994), no research hasbeen conducted on the Buwambo and similar kao-lin deposits. Kaolin from the Migade deposit wasselected for comparison purposes, as its kaolin isalso derived from weathering of the granite. TheMigade hill, on which the Migade kaolin depositis located, is about 2 km west of Buwambo. Theprimary focus of this study is to describe the oc-currence, and to present data for chemistry andmineralogy of the Buwambo kaolin deposit in or-der to evaluate the viability of the deposit. Possi-ble applications in industry are suggested.

INTRODUCTION

In the last decade, the kaolin industry has en-countered difficult times due to over-productionin Europe and the USA (Roskill, 1996). The emer-gence of major new high-quality kaolin sourcesin Brazil and Australia, and the constantly increas-ing supply of low-grade materials, has also af-fected the market. However, with increasing andnew industrial applications of kaolin, any strate-gically located promising deposit is worth exploit-ing, especially so in the developing world.

Kaolin is one of the most valuable, versatile,and widely used industrial minerals. Althoughmost important in papermaking, kaolin is usedextensively in the ceramics, rubber, paint, plas-tics, and pharmaceutical industries (Murray, 1991;Bundy, 1993). In Uganda, although kaolin depos-

246 G. W. A. Nyakairu et al.

GEOLOGY

The location of the Buwambo kaolin depositis shown in Fig. 1. The Buganda-Toro Systemcomprises a thin cover of overburden above thisdeposit. Rocks of the Buganda-Toro System, com-posed dominantly of cleaved, fine-grained sand-stone, slates, phyllites and schists, quartzites,

muscovite-biotite gneisses, and subordinate schistthat locally contain cordierite, overlie theBuwambo area. A large proportion of the rocks ofthe Buganda-Toro System, around the Buwamboarea, have been eroded, exposing Basement rocks,which consist of undifferentiated gneisses withsome later granites associated with somemigmatized Buganda-Toro rocks. The Buwambo

Fig. 1. Generalized geological map, showing the location of Buwambo and Migade kaolin deposits, after thegeological map of Uganda, Kampala sheet NA 36-14 (Geological Survey, Uganda, 1962). The inset is a map ofUganda with the general location of the clay deposits.

The Buwambo kaolin deposit in central Uganda 247

kaolin was, therefore, derived from weatheredgranites of the Basement that were exposed whenthe Buganda-Toro rocks were removed by erosion.Granites weather readily to kaolinite and quartzunder favorable conditions, which include highrainfall, rapid drainage, tropical climate, a lowwater table, and an adequate water supply to leachthe soluble components (Murray and Keller,1993). Kaolin is, in general, derived from alteredfeldspars and muscovite. Harris (1946) showedthat the rock from which the feldspar, which al-tered to the Buwambo kaolin, was a medium-grained granite, with 60 vol% feldspar. Thekaolinized rock has been discolored by groundwater and a relatively small proportion of it re-mains white.

Description of Buwambo kaolin depositThe Buwambo kaolin deposit is located about

27 km north of Kampala along the Kampala-Guluroad, less than 1 km to the east of the main roadwithin the Bombo area (Fig. 1). The deposit lieson top of the Buwambo hill Namakonkomi vil-

lage, at an average altitude of 1290 m. The kaolindeposit, an open cast trench, is situated some 12m below the summit on the slope west of the hill.The deposit is at longitude 32°31′54″ east and lati-tude 0°31′23″ north on map Series Y 732 Sheet61/III (Publ. by the Uganda Lands and SurveyDepartment, 1959). Presently, open pit-miningactivities are carried out below a 0.5–1.5-m-thicksoil cover. Kaolinitized rock is visible in an areaof at least 50 m by 100 m across, but it may covera much larger area. The kaolin reserve atBuwambo was estimated to be at least 50000 m3

by Pekkala and Katto (1994), but could be muchlarger.

Field observationThe Buwambo hill as a whole is a granite,

weathered and partly kaolinitized on top. As awhole, the occurrence is composed of quartz, kao-lin, feldspar, and muscovite, with secondary ironhydroxide staining in fissures and cavities, whichare more frequent within the topmost 2 m. Mus-covite mica is scattered through the kaolin in tiny

Fig. 2. Representative X-ray diffraction patterns of bulk Buwambo kaolin sample (BW-1) in comparison withkaolin from the Migade (MG-1) deposit. K = kaolinite; Q = quartz; I/M = muscovite/illite and F = feldspar. Thesamples are mainly composed of kaolinite, quartz, and muscovite/illite.

248 G. W. A. Nyakairu et al.

Tabl

e 1.

Che

mic

al a

nd s

emi-

quan

tita

tive

min

eral

ogic

al c

ompo

siti

ons

of k

aoli

ns f

r om

the

Buw

ambo

and

Mig

ade

depo

sits

, cen

tral

Uga

nda.

The

min

eral

ogic

al c

ompo

siti

on c

alcu

lati

ons

wer

e pe

rfor

med

fro

m X

RD

spe

ctra

usi

ng a

mod

ifie

d m

etho

d af

ter

Schu

ltz

(196

4).

Maj

or e

lem

e nt

data

in

wt.

%;

min

e ral

ogic

al c

ompo

siti

on i

n w

t.%

; al

l F

e re

port

e d a

s F

e 2O

3; L

OI

= l

oss

on i

gnit

ion;

dat

a fo

r U

.K.

(Eng

land

) k a

olin

fro

m M

urra

y an

d K

e lle

r(1

993)

; pu

re k

aoli

n da

ta f

rom

New

man

and

Bro

wn

(198

7).

The Buwambo kaolin deposit in central Uganda 249

flakes. Quartz also occurs in veins, 2–10 cm wideand as lenses up to 0.5 m across, it is mostlygrayish in color. The kaolin body is intimatelypenetrated by reddish veins, which probably markgroundwater passages.

SAMPLES AND METHODS

For this study, 12 kaolin samples fromBuwambo and 6 from Migade were analyzed. SixBuwambo and three Migade samples were selectedfor X-ray fluorescence (XRF) spectrometry analy-sis. The samples were collected at an averagedepth of 4 m from the top of the open pit and at aregular spacing of a few meters horizontally. Sam-ple preparation, as well as major and trace ele-ment analysis, by X-ray fluorescence spectrometryand instrumental neutron activation analysis, andmineralogical studies by X-ray diffraction, weredone as described by Nyakairu and Koeberl(2001).

RESULTS

MineralogyThe main minerals identified by XRD in the

kaolin deposits are kaolinite [Al2Si2O5(OH)4],quartz and illite/muscovite, with minor amountsof feldspars (Fig. 2; Table 1). Kaolinite alone com-prises about 74–93 wt.% of all analyzed samples.The similarity in mineralogical composition of theBuwambo and Migade kaolins shows that thesetwo deposits were probably derived from the samesource rocks, and also may indicate a similar ex-tent of kaolinization. The low feldspar contentshows the weathering extent source rocks haveundergone (cf., Mongelli and Moresi, 1990).

GeochemistryMajor elements Table 1 shows the chemical com-position of selected kaolin samples from theBuwambo and Migade deposits. For comparisoncompositional data of kaolin from the U.K.

Fig. 3. Plots of: (a), (b) major element oxides; (c), (d) trace elements of Uganda kaolin samples normalized toSingo granite (data from Nagudi et al., 2000). The kaolins all show alumina enrichment and depletion in the otheroxides. Note the Ni enrichment in all kaolins.

250 G. W. A. Nyakairu et al.

(Murray and Keller, 1993) are included. The Singogranite (Nagudi et al., 2000) occurs in westerncentral Uganda and was derived from the Base-ment rocks, hence, is assumed to be similar incomposition to the granite rocks from which thekaolins studied were derived, and is the only suchrock formation in Uganda for which detailed ma-jor and trace element data are available. The ma-jor element distribution reflects the mineralogy ofthe kaolin samples. The kaolin samples, are char-acterized by low SiO2 (values ranging between46.4 to 52.4 wt.%) and high Al2O3 (values rang-ing from 32.7 to 38.3 wt.%). The Fe2O3, MnO,MgO, and K2O contents of the Buwambo kaolinare similar to those of the Migade and U.K.kaolins. The presence of minor amounts of CaOand Na2O in the U.K. kaolin indicates that theBuwambo kaolin is more weathered.

Relative to the Singo granite, SiO2, MnO,MgO, and, especially TiO2 and Fe2O3, are de-

pleted, and Al2O3 is enriched in the kaolins (Figs.3(a) and (b)). Sodium and CaO are completely lostfrom the Buwambo and Migade deposits, whereasK2O is only moderately depleted and correlateswith the illite content.Trace elements Compared to the Singo granite,the trace element abundances show variability andsome depletion (Figs. 3(c) and (d)). Scandium, Cr,Co, Zn, Rb, Sr, Y, Zr, Nb, Sb, Cs, Ba, and U showconcentrations below the abundances for the Singogranite, probably due to weathering andkaolinization processes. This is in agreement withstudies by Nesbitt et al. (1980) and Wronkiewiczand Condie (1987), who conclude that smallercations, such as Na, Ca, and Sr, are selectivelyleached from weathering profiles, whereas cati-ons with relatively large ion radii, such as K, Cs,Rb, and Ba, have been fixed by preferential ex-change and adsorption on clays. Chromium, Co,Ni, V, Sc, and Cs are mainly concentrated in clay

Fig. 4. Rare earth element plots of Uganda kaolin samples normalized to (a), (b): Singo granite, data fromNagudi et al. (2000) and (c), (d): C1 chondrite data from Taylor and McLennan (1985). The kaolins show ageneral LREE enrichment with a Ce negative anomaly.

The Buwambo kaolin deposit in central Uganda 251

Tabl

e 2.

Tr

ace

elem

ent

com

posi

tion

of

kaol

ins

from

Buw

ambo

and

Mig

ade

depo

sits

, ce

ntra

l U

gand

a

Trac

e e l

e me n

t da

ta i

n pp

m;

Sing

o gr

anit

e da

ta f

rom

Nag

udi

e t a

l. (

2000

); n

.d. =

not

de t

e rm

ine d

; C

e /C

e * =

Ce c

n/((

La c

n2/3 )·

(Nd c

n1/

3 )) w

here

sub

scri

pt c

n =

cho

ndri

te n

orm

aliz

e d.

252 G. W. A. Nyakairu et al.

minerals, whereas Ni, Co, and especially Cr otherfactors may control their distribution (Nesbitt,1979). Nickel is enriched in all the kaolin sam-ples. In terms of trace element contents, the twokaolin deposits are very similar. The differencesbetween the trace element contents in theBuwambo and Migade deposits are so small thatit seems that the deposits were derived from par-ent rocks with similar chemical and mineralogi-cal compositions.

The effects of kaolinization and weathering areevident in the variable total REE contents of thekaolins (Table 2). The REE abundances normal-ized to Singo granite and chondritic values areshown in Figs. 4(a)–(d). The kaolins in generalshow uniform REE patterns, being more enrichedin LREE compared to the HREE. The kaolin sam-ples show a distinct negative Ce anomaly, whichis a result of weathering (see below).

SUITABILITY FOR CERAMIC APPLICATIONS

An assessment of possible ceramic applicationsof Buwambo and Migade kaolins, was carried outby comparing its composition with the kaolinscommercially marketed in Europe (cf., Ligas etal., 1997). Figure 5 shows a comparison of the

Ugandan kaolin major element composition withthose of commercially used kaolins from Europe.This plot indicates that the Uganda kaolins have ahigher sand component compared to those fromLatium (Italy). In contrast, the mineralogical com-position diagram (Fig. 6) shows a closer similar-ity to high quality kaolins from Germany and theU.K. This comparison indicates that the Ugandakaolins have a potential for high quality indus-trial use.

DISCUSSION

The mobility of chemical elements in naturalenvironments depends largely on their redox po-tential and the pH of water. Chemical weathering,such as kaolinization of rocks, usually starts in aweakly acidic environment, but liberation of al-kalis and alkali earths’ results in a rapid increaseof the pH. Alkalis and alkali earths are quicklyremoved from the kaolinization environment, thusthe last stages of kaolinization proceed in acidenvironment. This change, from slightly acid tostrongly alkaline, and finally to acid conditionsis, of primary importance for the migration of sev-eral chemical elements in the course ofkaolinization. Progressive weathering and

Fig. 6. Ternary diagram: quartz/kaolinite/other min-erals for the Uganda kaolins compared with the kaolinscommercially marketed in Europe. The Uganda kaolinsfall in the field of the Bretagne and Bavaria kaolins.

Fig. 5. Ternary SiO2/Al2O3/total oxides diagram ofUganda kaolins compared to several European com-mercially marketed kaolins (data from Ligas et al.,1997).

The Buwambo kaolin deposit in central Uganda 253

kaolinization of the granite lead to decompositionof all minerals except quartz in the Buwambo de-posit. The decrease in the SiO2 content relative tothe Singo granite shows an advanced stage ofkaolinization for the Buwambo kaolinite, as thesolubility of quartz increases in an alkaline envi-ronment (Garbarino et al., 1994). Successive de-creases in the SiO2, MnO, MgO, P2O5, and moreso in the Fe2O3, TiO2, CaO, Na2O, and K2O con-centrations shows their higher mobility. The re-duction in these oxides is compensated by enrich-ment in Al2O3 and higher LOI.

The presence of TiO2, Fe2O3, and MnO has anegative effect on the quality of Buwambo kao-lin. In kaolins, Ti occurs in small amounts, mainlyin the form of anatase, which in finely dispersedform is difficult to determine by XRD (Fischer,1984). However, during weathering andkaolinization small amounts of Ti enter thekaolinite structure (Murray and Keller, 1993) andit enhances the coloring effect of iron in kaolins(Fischer, 1984). In the Buwambo kaolin, iron oc-curs in form of hydrated oxides adsorbed on smallkaolin grains and in secondary iron mineralsformed in the course of weathering andkaolinization. In small amounts iron enters thekaolinite structure and replace aluminum in theoctahedral layer (Murray and Keller, 1993). TheMnO content in Buwambo kaolin may be corre-lated to biotite, hornblende, and, to some extentin other iron-bearing minerals. During weather-ing and kaolinization, Mn is easily oxidized toMn4+ and immobilized in the form of oxides, alongwith the rise in pH. In a high reducing environ-ment, Mn enters the structure of carbonates, inwhich Mn2+ may replace Fe2+. However, in an acidenvironment, Mn is leached during kaolinization.

Similarly, trace element contents in bothMigade and Buwambo kaolins decrease with in-creasing extent of weathering and kaolinization(Fig. 3) compared to the Singo granite. The higherconcentration of Ni in the kaolin samples may bedue to the mobilization of iron oxide. Trace ele-ments are liberated from primary structures dur-ing granite weathering and kaolinization processesand are subsequently removed from the rocks.

Leaching of these elements in Buwambo depositproceeded at varying rates, depending on the re-sistance of the elements-bearing minerals toweathering, kaolinization, and on physico-chemi-cal conditions (e.g., pH, Eh). Assuming that weath-ering proceeded under similar chemical conditionsin the Buwambo and Migade deposits, the differ-ence in the elements composition in the kaolindeposits resulted by kaolinization process. Stron-tium is more concentrated than Ba in all samples.Strontium is mostly bound in plagioclase, whereit replaces Ca. Barium has a behavior similar tothat of K, and is mostly concentrated in K-feldspars and micas. Both Ba and Sr are easilymobilized during weathering and are readily re-moved from the environment, but leaching of Srstarts much earlier and is more intense. Calciumand K are almost completely removed by weath-ering, but significant amounts of Sr and Ba, whichhave a close affinity to these elements, remain inthe kaolin. Presumably Sr and Ba are partlyadsorbed by clay minerals or were added to a smallextent by downward percolating solutions.

Potassium, Rb, and Cs are mainly hosted inmicas and K-feldspar (Heier and Billings, 1970),thus alteration of these minerals will dominate thefractionation of these elements. Rubidium has atrend comparable with that of Nb and V and it isless mobile than K, whose behavior it generallyfollows. This has been explained in that Rb, incontrast to K, is preferentially retained in weath-ered illite (Garrels and Christ, 1965), but Nb andV are less mobile (Middelburg et al., 1988). Theconcentration of these elements in the Buwambokaolin is controlled by the phyllosilicates. Fengand Kerrich (1990) noted that Cr, Co, Ni, Ti, andV behave similarly during magmatic processes,but cautioned that they may be fractionated dur-ing weathering. The high Ni content in the kaolinsis related to its being easily mobilized duringweathering (cf., Turekian, 1978). Scandium hasbeen slightly concentrated in the kaolin duringkaolinization compared to other trace elements.The tendency of Cr to be removed is probably re-lated to Cr4+ oxidation, soluble as chromate ion(Marsh, 1991). Thorium and U behave differently

254 G. W. A. Nyakairu et al.

during weathering, in that U, unlike Th, is chemi-cally mobile as U6+ and is precipitated as insolu-ble U4+ compound. Thorium and U are mainlyaccommodated in some heavy minerals, such aszircon, which are resistant to weathering. The lowtrace elements contents in the Buwambo kaolinsamples in comparison to Singo granite can beaccounted by their clay mineralogy with notice-able quantities of kaolinite, in addition to the factthat these elements are preferentially fixed by illiteas K2O reveals. However, the kaolinite percent-ages in kaolin samples constitute an additionalevidence of intensity of weathering andkaolinization extent.

The Singo granite-normalized REE patterns(Fig. 4) show LREE enrichment, a slight positiveEu and a negative Ce anomalies, and HREE de-pletion in the Buwambo kaolin with the excep-tion of BW-3 and BW-9 samples showing highercontents. Compared to Buwambo kaolin, Migadekaolin shows no Eu anomaly. The chondrite-nor-malized REE patterns of the kaolins are identicalwith LREE enrichment, a negative Ce anomalyand HREE depletion with samples BW-3 and BW-9 showing higher contents. However, Migade kao-lin shows more REE contents than Buwambo sam-ples. The LREE enrichment reflects the results ofextreme weathering while the slight positive Euanomaly is due to plagioclase or orthoclase pres-ence in the kaolin samples. Weathering causesfractionation between LREE-HREE and betweenCe and other REE. The REE are released fromprimary minerals and taken up by the secondaryphases during kaolinization. The REE are leachedby local meteoric waters from the upper levels ofthe profiles and concentrated in deeper zones,probably in the form of complexes, or by mechani-cal transport of secondary phases (clay mineralsand oxides) (cf., Prudencio et al., 1989). However,Ce has a different behavior, as it may be partlyoxidized to Ce4+, which is retained in the upperlevels of the soil profile relative to the other tri-valent REE. This accounts for the observed nega-tive Ce anomalies displayed in the Buwambo kao-lin samples. Although kaolinite accommodates theREE (Prudencio et al., 1989), and particularly the

LREE (Nesbitt, 1979), a basic pH environment isalso known to fractionate LREE-HREE, as theHREE are preferentially retained in solution,where they form soluble complexes (Cantrell andByrne, 1987).

However, some trace elements and the LREEsare extremely concentrated in the kaolin samples.For example, BW-3 and BW-9 shows extraordi-narily high concentrations of Sr, Ba, Zr, Y, LREE,and P2O5. The high concentrations of the LREEsand P2O5 in these samples strongly suggest con-trol by phosphate minerals, such as apatite andmonazite, during weathering and kaolinizationprocesses. The high Zr and Y contents are associ-ated with heavy minerals such as zircon althoughthe zircon content hardly explains the low abun-dance of Hf. The high level of Sr in the samples isdue to the uptake of Sr in magnesite and arago-nite lattice. High Ba content is due to the pres-ence of feldspar and clay contents. However, theirdistributions are not only controlled by zircon, clayminerals, or phosphates, but a rather combinationof these minerals. The LREEs enrichments in theMigade kaolin show that the Migade kaolin hasundergone more weathering and has beenkaolinitized to a greater extent.

CONCLUSIONS

Kaolinite is the dominant mineral in theBuwambo and Migade kaolins, followed byquartz, with muscovite/illite as accessory miner-als. The chemical composition of the kaolin isessentially SiO2 and Al2O3, with minor amountsof TiO2, Fe2O3, MnO, MgO, K2O, and P2O5. Thedecrease in SiO2 content relative to the Singo gran-ite indicates kaolinization in alkaline environment,and Al2O3 has remained immobile. As TiO2, Fe2O3,MnO, and MgO are known to negatively affectthe kaolin color, the Buwambo and Migade kaolinshave almost a white color. The loss in MgO, CaO,Na2O, and K2O indicate the extent to whichkaolinization of the Buwambo kaolin has under-gone. The kaolin samples from Buwambo are quitepure and are virtually equal to the chemical com-position of pure kaolinite. From chemical and

The Buwambo kaolin deposit in central Uganda 255

mineralogical composition, Buwambo and Migadekaolins are suitable for the production of earthen-ware, sanitary, white ware, and electrical insula-tors production.

Compared to average source rock abundances,e.g., the Singo granite, the Buwambo and Migadekaolins are depleted in all trace elements and onlyNi shows an enrichment. Assuming uniformweathering conditions, Ni has been concentratedby the kaolinization process. The Singo granitenormalized REE patterns shows LREE enrich-ment, with a slight positive Eu and a negative Ceanomalies for the Buwambo kaolin. The chondritenormalized REE patterns, however, show littlechange, except a slight LREE enrichment and anegative Cerium anomaly. The Migade kaolin hasundergone a higher degree of weathering andkaolinization than the Buwambo kaolin.

Acknowledgments—The authors are grateful to theAustrian Academic Exchange Service (ÖAD), whichprovided a Ph.D. stipend and financial assistance forfieldwork to G.W.A.N. We acknowledge W. U. Reimold(University of Witwatersrand, Johannesburg) for XRFanalyses, and S. Gier (Institute of Petrology, ViennaUniversity, Vienna) for assistance with XRD analysis.The laboratory work was supported in part by the Aus-trian FWF, project Y58-GEO (to C.K.). We are grate-ful to Profs. K. Suzuki and J. Matsuda, as well as twoanonymous reviewers, for constructive comments onthe manuscript.

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