Geochemical Journal, Vol. 35, pp. 245 to 256, 2001*Corresponding autho - PDF document

Geochemical Journal, Vol. 35, pp. 245 to 256, 2001*Corresponding author (e-mail: christian.koeberl@univie.ac.at) W. A. NYAKAIRU* and HInstitute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, AustriaInstitute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, AustriaReceived February 12, 2001; Accepted June 20, 2001for 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, with and Al, with the other oxides being present in trace amounts. The depletion in Ti, Fe, Mn, Mg,white 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-its occur, their industrial use is restricted to manu-Kaahwa, 1998, and references therein). However,small quantities of kaolin are locally used as whitelin deposits. Kaolin from the Migade deposit wasalso derived from weathering of the granite. Theis located, is about 2 km west of Buwambo. Themineralogy of the Buwambo kaolin deposit in or-countered difficult times due to over-productionin Europe and the USA (Roskill, 1996). The emer-in Brazil and Australia, and the constantly increas-fected the market. However, with increasing andand widely used industrial minerals. Althoughextensively in the ceramics, rubber, paint, plas-tics, and pharmaceutical industries (Murray, 1991;Bundy, 1993). In Uganda, although kaolin depos- 246G. W. A. Nyakairu is shown in Fig. 1. The Buganda-Toro Systemdeposit. Rocks of the Buganda-Toro System, com-that 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 withmigmatized 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 of the Buganda-Toro rocks were removed by erosion.the soluble components (Murray and Keller,1993). Kaolin is, in general, derived from alteredthat 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 ground27 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 lieslage, at an average altitude of 1290 m. The kaolinm below the summit on the slope west of the hill. east and lati- north on map Series Y 732 SheetDepartment, 1959). Presently, open pit-miningactivities are carried out below a 0.5Ð1.5-m-thicksoil cover. Kaolinitized rock is visible in an areaa much larger area. The kaolin reserve atlarger.weathered and partly kaolinitized on top. As awhole, the occurrence is composed of quartz, kao-lin, feldspar, and muscovite, with secondary ironare more frequent within the topmost 2 m. Mus-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. 248G. W. A. Nyakairu Table 1. Chemical and semi-quantitative mineralogical compositions of kaolins fre performed from XRD spectra using a modified method after Schultz (1964).eported as Feom Murray and Keller(1993); pure kaolin data from Newman and Br flakes. Quartz also occurs in veins, 2Ð10 cm widegrayish in color. The kaolin body is intimatelyFor this study, 12 kaolin samples fromBuwambo and 6 from Migade were analyzed. Sixsis. 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-mineralogical studies by X-ray diffraction, weredone as described by Nyakairu and KoeberlESULTSTS2Si2O5(OH)4],quartz and illite/muscovite, with minor amountsof feldspars (Fig. 2; Table 1). Kaolinite alone com-prises about 74Ð93 wt.% of all analyzed samples.tent of kaolinization. The low feldspar contentshows the weathering extent source rocks have Table 1 shows the chemical com-position of selected kaolin samples from theFig. 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 other 250G. W. A. Nyakairu (Murray and Keller, 1993) are included. The Singocentral Uganda and was derived from the Base-composition to the granite rocks from which thejor and trace element data are available. The ma-the kaolin samples. The kaolin samples, are char- (values ranging between46.4 to 52.4 wt.%) and high Al (values rang-ing from 32.7 to 38.3 wt.%). The FeMgO, and Kkaolins. The presence of minor amounts of CaOMgO, and, especially TiO and Fepleted, and Al is enriched in the kaolins (Figs.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 showkaolinization processes. This is in agreement with. (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,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. Table 2. Trace element composition of kaolins frTrace element data in ppm; Singo granite data fr)) where subscript cn = chondrite normalized. 252G. W. A. Nyakairu factors may control their distribution (Nesbitt,kaolin deposits are very similar. The differencesit seems that the deposits were derived from par-The effects of kaolinization and weathering arekaolins (Table 2). The REE abundances normal-shown in Figs. 4(a)Ð(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, whichUITABILITYPPLICATIONSthose of commercially used kaolins from Europe.position diagram (Fig. 6) shows a closer similar-ity to high quality kaolins from Germany and theU.K. This comparison indicates that the Ugandaenvironments depends largely on their redox po-tential and the pH of water. Chemical weathering,of the pH. Alkalis and alkali earths are quicklyremoved from the kaolinization environment, thusenvironment. This change, from slightly acid toFig. 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 SiOUganda kaolins compared to several European com-mercially marketed kaolins (data from Ligas et al., posit. The decrease in the SiO content relative to, MnO, MgO, P, TiOO, and Kcentrations shows their higher mobility. The re-ment in AlThe presence of TiOnegative 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 smallkaolinite structure and replace aluminum in theoctahedral layer (Murray and Keller, 1993). Thein other iron-bearing minerals. During weather- and immobilized in the form of oxides, along may replace Fe. However, in an acidenvironment, Mn is leached during kaolinization.Similarly, trace element contents in both(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-and are subsequently removed from the rocks.sistance of the elements-bearing minerals tocal conditions (e.g., pH, Eh). Assuming that weath-in the Buwambo and Migade deposits, the differ-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-moved from the environment, but leaching of Srhave a close affinity to these elements, remain inthe kaolin. Presumably Sr and Ba are partlyfractionation of these elements. Rubidium has atrend comparable with that of Nb and V and it isless mobile than K, whose behavior it generallyV are less mobile (Middelburg ., 1988). Theand Kerrich (1990) noted that Cr, Co, Ni, Ti, andbut cautioned that they may be fractionated dur-weathering (cf., Turekian, 1978). Scandium has oxidation, soluble as chromate ion(Marsh, 1991). Thorium and U behave differently 254G. W. A. Nyakairu during weathering, in that U, unlike Th, is chemi- compound. Thorium and U are mainlyzircon, which are resistant to weathering. The lowO reveals. However, the kaolinite percent-tion of BW-3 and BW-9 samples showing higherkaolin shows no Eu anomaly. The chondrite-nor-and HREE depletion with samples BW-3 and BW-9 showing higher contents. However, Migade kao-ples. The LREE enrichment reflects the results ofence in the kaolin samples. Weathering causesCe and other REE. The REE are released fromphases during kaolinization. The REE are leachedby local meteoric waters from the upper levels of., 1989). However,Ce has a different behavior, as it may be partlyvalent REE. This accounts for the observed nega-lin samples. Although kaolinite accommodates theLREE (Nesbitt, 1979), a basic pH environment iswhere they form soluble complexes (Cantrell andHowever, some trace elements and the LREEsFor example, BW-3 and BW-9 shows extraordi-narily high concentrations of Sr, Ba, Zr, Y, LREE,. The high concentrations of the LREEs in these samples strongly suggest con-processes. The high Zr and Y contents are associ-the zircon content hardly explains the low abun-dance of Hf. The high level of Sr in the samples isence of feldspar and clay contents. However, theirof these minerals. The LREEs enrichments in theundergone more weathering and has beenquartz, with muscovite/illite as accessory miner-als. The chemical composition of the kaolin is and Alof TiO content relative to the Singo gran- has remained immobile. As TiOMnO, and MgO are known to negatively affectthe kaolin color, the Buwambo and Migade kaolinshave almost a white color. The loss in MgO, CaO,kaolinization of the Buwambo kaolin has under-gone. The kaolin samples from Buwambo are quite ware, sanitary, white ware, and electrical insula-Ni shows an enrichment. Assuming uniformweathering conditions, Ni has been concentratedby the kaolinization process. The Singo graniteanomalies for the Buwambo kaolin. The chondritenormalized REE patterns, however, show littlenegative Cerium anomaly. The Migade kaolin hasundergone a higher degree of weathering andAustrian Academic Exchange Service (…AD), whichfieldwork 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-Bundy, W. M. (1993) The diverse industrial applica-(Murray, H. H., Bundy, W. and Harvey, C., eds.), ., Spec. Publ. 1, Boulder, 43Ð47.grained clastic sediments in the Archean AbitibiFischer, P. (1984) Some comments on the color of firedErde. Freeman and Cooper, San Fran-Geological Survey, Uganda (1962) Geological map ofHarris, N. (1946) A note on kaolin supply near Bombo.Heier, K. S. and Billings, G. K. (1970) Rubidium. book of Geochemistry (Wedepohl, K. H., ed.), 37B1Ð37N1, Springer, Berlin.Ligas, P., Uras, I., Dondi, M. and Marsigli, M. (1997)Kaolinitic materials from Romana (north-west Sar-Middelburg, J. J., van der Weijden, C. H. and Woittiez,J. R. W. (1988) Chemical processes affecting themobility of major, minor, and trace elements duringMongelli, G. (1995) Trace elements distribution andtion of shales from southern Apennines (Italy). eral. Petrol, 101Ð114.Miner. Petrogr. ActaMurray, H. H. (1991) Overview: Clay mineral applica-Murray, H. H. and Keller, W. D. (1993) Kaolins, kaolins (Murray,H. H., Bundy, W. and Harvey, C., eds.), ., Spec. Publ. 1, Boulder, 1Ð24.J. Afr.Nesbitt, H. W. (1979) Mobility and fractionation of rareNatureNesbitt, H. W., Markovics, G. and Price, R. C. (1980)Chemical processes affecting alkalis and alkali earthsNewman, A. C. D. and Brown, G. (1987) The chemical(Newman, A. C. D., ed.), Monograph-Mineralogical Society, 6, 1Ð128, Mineralogical So-ciety, London, U.K. 256G. W. A. Nyakairu Nyakairu, G. W. A. and Kaahwa, Y. (1998) Phase tran-Amer. Ceram. Soc. BullNyakairu, G. W. A. and Koeberl, C. (2001) Minera-Pekkala, Y. and Katto, E. (1994) Reconnaissance fieldtrip to the Namasera and Buwambo kaolin occur-P. (1989) Rare earth distribution and its correlationmineralogical composition from X-ray and chemicalU.S. Geol. Surv. Prof. PapTaylor, S. R. and McLennan, S. H. (1985) Turekian, K. K. (1978) Nickel-Behavior during weath-Handbook of Geochemistry (Wedepohl, K. H.,ed.), Vol. II, Sect. 28-G-1, Springer, Berlin.Wronkiewicz, D. J. and Condie, K. C. (1987)Geochemistry of Archean shales from theWitwatersrand Supergroup, South Africa: source-area