NOTE the Buwambo Kaolin Deposit in Central Uganda: Mineralogical And

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

NOTE

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, Austria 2Institute 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 analyzed for its mineralogical and chemical composition. The kaolin is derived from granite of the basement, which is exposed due to deeply weathered Buganda-Toro cover rocks. Kaolinite is the dominant mineral, with quartz and muscovite/illite as accessory minerals. Chemical data show that the kaolin is composed mainly

of 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 almost white color. The kaolinization and weathering processes have enriched Ni and depleted other trace element contents in the Buwambo kaolin. The chondrite-normalized rare earth element (REE) patterns show enrichment in the light REEs, with a negative Ce anomaly. The REE pattern and the content of the other trace elements, show evidence of alteration and weathering processes related to kaolinization. The mineralogical and chemical compositions indicate that the kaolin is suitable for industrial use.

its occur, their industrial use is restricted to manu- INTRODUCTION facturing of bricks and pottery (Nyakairu and In the last decade, the kaolin industry has en- Kaahwa, 1998, and references therein). However, countered difficult times due to over-production small quantities of kaolin are locally used as white in Europe and the USA (Roskill, 1996). The emer- wash or “enoni” on the mud and wattle dwellings. gence of major new high-quality kaolin sources Apart from reconnaissance work by Harris (1946) in Brazil and Australia, and the constantly increas- and Pekkala and Katto (1994), no research has ing supply of low-grade materials, has also af- been conducted on the Buwambo and similar kao- fected the market. However, with increasing and lin deposits. Kaolin from the Migade deposit was new industrial applications of kaolin, any strate- selected for comparison purposes, as its kaolin is gically located promising deposit is worth exploit- also derived from weathering of the granite. The ing, especially so in the developing world. Migade hill, on which the Migade kaolin deposit Kaolin is one of the most valuable, versatile, is located, is about 2 km west of Buwambo. The and widely used industrial minerals. Although primary focus of this study is to describe the oc- most important in papermaking, kaolin is used currence, and to present data for chemistry and extensively in the ceramics, rubber, paint, plas- mineralogy of the Buwambo kaolin deposit in or- tics, and pharmaceutical industries (Murray, 1991; der to evaluate the viability of the deposit. Possi- Bundy, 1993). In Uganda, although kaolin depos- ble applications in industry are suggested.

*Corresponding author (e-mail: [email protected])

245 246 G. W. A. Nyakairu et al.

muscovite-biotite gneisses, and subordinate schist GEOLOGY that locally contain cordierite, overlie the The location of the Buwambo kaolin deposit Buwambo area. A large proportion of the rocks of is shown in Fig. 1. The Buganda-Toro System the Buganda-Toro System, around the Buwambo comprises a thin cover of overburden above this area, have been eroded, exposing Basement rocks, deposit. Rocks of the Buganda-Toro System, com- which consist of undifferentiated gneisses with posed dominantly of cleaved, fine-grained sand- some later granites associated with some stone, slates, phyllites and schists, quartzites, migmatized Buganda-Toro rocks. The Buwambo

Fig. 1. Generalized geological map, showing the location of Buwambo and Migade kaolin deposits, after the geological map of Uganda, Kampala sheet NA 36-14 (Geological Survey, Uganda, 1962). The inset is a map of Uganda with the general location of the clay deposits. The Buwambo kaolin deposit in central Uganda 247 kaolin was, therefore, derived from weathered lage, at an average altitude of 1290 m. The kaolin granites of the Basement that were exposed when deposit, an open cast trench, is situated some 12 the Buganda-Toro rocks were removed by erosion. m below the summit on the slope west of the hill. Granites weather readily to kaolinite and quartz The deposit is at longitude 32°31′54″ east and lati- under favorable conditions, which include high tude 0°31′23″ north on map Series Y 732 Sheet rainfall, rapid drainage, tropical climate, a low 61/III (Publ. by the Uganda Lands and Survey water table, and an adequate water supply to leach Department, 1959). Presently, open pit-mining the soluble components (Murray and Keller, activities are carried out below a 0.5Ð1.5-m-thick 1993). Kaolin is, in general, derived from altered soil cover. Kaolinitized rock is visible in an area feldspars and muscovite. Harris (1946) showed of at least 50 m by 100 m across, but it may cover that the rock from which the feldspar, which al- a much larger area. The kaolin reserve at tered to the Buwambo kaolin, was a medium- Buwambo was estimated to be at least 50000 m3 grained granite, with 60 vol% feldspar. The by Pekkala and Katto (1994), but could be much kaolinized rock has been discolored by ground larger. water and a relatively small proportion of it re- mains white. Field observation The Buwambo hill as a whole is a granite, Description of Buwambo kaolin deposit weathered and partly kaolinitized on top. As a The Buwambo kaolin deposit is located about whole, the occurrence is composed of quartz, kao- 27 km north of Kampala along the Kampala-Gulu lin, feldspar, and muscovite, with secondary iron road, less than 1 km to the east of the main road hydroxide staining in fissures and cavities, which within the Bombo area (Fig. 1). The deposit lies are more frequent within the topmost 2 m. Mus- on top of the Buwambo hill Namakonkomi vil- 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 with kaolin from the Migade (MG-1) deposit. K = kaolinite; Q = quartz; I/M = muscovite/illite and F = feldspar. The samples are mainly composed of kaolinite, quartz, and muscovite/illite. 248 G. W. A. Nyakairu et al.

om Murray and Keller om Murray

om the Buwambo and Migade deposits, central Uganda. The

; LOI = loss on ignition; data for U.K. (England) kaolin fr

eported as Fe

e performed from XRD spectra using a modified method after Schultz (1964). e performed from

own (1987).

Table 1. Chemical and semi-quantitative mineralogical compositions of kaolins fr Table mineralogical composition calculations wer

Major element data in wt.%; mineralogical composition in wt.%; all Fe r (1993); pure kaolin data from Newman and Br kaolin data from (1993); pure The Buwambo kaolin deposit in central Uganda 249 flakes. Quartz also occurs in veins, 2Ð10 cm wide RESULTS and as lenses up to 0.5 m across, it is mostly grayish in color. The kaolin body is intimately Mineralogy penetrated by reddish veins, which probably mark The main minerals identified by XRD in the groundwater passages. kaolin deposits are kaolinite [Al2Si2O5(OH)4], quartz and illite/muscovite, with minor amounts of feldspars (Fig. 2; Table 1). Kaolinite alone com- SAMPLES AND METHODS prises about 74Ð93 wt.% of all analyzed samples. For this study, 12 kaolin samples from The similarity in mineralogical composition of the Buwambo and 6 from Migade were analyzed. Six Buwambo and Migade kaolins shows that these Buwambo and three Migade samples were selected two deposits were probably derived from the same for X-ray fluorescence (XRF) spectrometry analy- source rocks, and also may indicate a similar ex- sis. The samples were collected at an average tent of kaolinization. The low feldspar content depth of 4 m from the top of the open pit and at a shows the weathering extent source rocks have regular spacing of a few meters horizontally. Sam- undergone (cf., Mongelli and Moresi, 1990). ple preparation, as well as major and trace element analysis, by X-ray fluorescence spectrometry Geochemistry and instrumental neutron activation analysis, and Major elements Table 1 shows the chemical com- mineralogical studies by X-ray diffraction, were position of selected kaolin samples from the done as described by Nyakairu and Koeberl Buwambo and Migade deposits. For comparison (2001). compositional 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 to Singo granite (data from Nagudi et al., 2000). The kaolins all show alumina enrichment and depletion in the other oxides. Note the Ni enrichment in all kaolins. 250 G. W. A. Nyakairu et al.

(Murray and Keller, 1993) are included. The Singo pleted, and Al2O3 is enriched in the kaolins (Figs. granite (Nagudi et al., 2000) occurs in western 3(a) and (b)). Sodium and CaO are completely lost central Uganda and was derived from the Base- from the Buwambo and Migade deposits, whereas ment rocks, hence, is assumed to be similar in K2O is only moderately depleted and correlates composition to the granite rocks from which the with the illite content. kaolins studied were derived, and is the only such Trace elements Compared to the Singo granite, rock formation in Uganda for which detailed ma- the trace element abundances show variability and jor and trace element data are available. The ma- some depletion (Figs. 3(c) and (d)). Scandium, Cr, jor element distribution reflects the mineralogy of Co, Zn, Rb, Sr, Y, Zr, Nb, Sb, Cs, Ba, and U show the kaolin samples. The kaolin samples, are char- concentrations below the abundances for the Singo acterized by low SiO2 (values ranging between granite, probably due to weathering and 46.4 to 52.4 wt.%) and high Al2O3 (values rang- kaolinization processes. This is in agreement with ing from 32.7 to 38.3 wt.%). The Fe2O3, MnO, studies by Nesbitt et al. (1980) and Wronkiewicz MgO, and K2O contents of the Buwambo kaolin and Condie (1987), who conclude that smaller are similar to those of the Migade and U.K. cations, such as Na, Ca, and Sr, are selectively kaolins. The presence of minor amounts of CaO leached from weathering profiles, whereas cati- and Na2O in the U.K. kaolin indicates that the ons with relatively large ion radii, such as K, Cs, Buwambo kaolin is more weathered. Rb, and Ba, have been fixed by preferential ex-

Relative to the Singo granite, SiO2, MnO, change and adsorption on clays. Chromium, Co, MgO, and, especially TiO2 and Fe2O3, are de- 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 from Nagudi et al. (2000) and (c), (d): C1 chondrite data from Taylor and McLennan (1985). The kaolins show a general LREE enrichment with a Ce negative anomaly. The Buwambo kaolin deposit in central Uganda 251

)) where subscript cn = chondrite normalized. )) where

1/3

)á(Nd

2/3

/((La

om Buwambo and Migade deposits, central Uganda

om Nagudi et al. (2000); n.d. = not determined; Ce/Ce* = Ce

Trace element data in ppm; Singo granite data fr Trace

Table 2. Trace element composition of kaolins fr 2. Trace Table 252 G. W. A. Nyakairu et al.

Fig. 6. Ternary diagram: quartz/kaolinite/other min-

Fig. 5. Ternary SiO2/Al2O3/total oxides diagram of erals for the Uganda kaolins compared with the kaolins Uganda kaolins compared to several European com- commercially marketed in Europe. The Uganda kaolins mercially marketed kaolins (data from Ligas et al., fall in the field of the Bretagne and Bavaria kaolins. 1997).

minerals, whereas Ni, Co, and especially Cr other Ugandan kaolin major element composition with factors may control their distribution (Nesbitt, those of commercially used kaolins from Europe. 1979). Nickel is enriched in all the kaolin sam- This plot indicates that the Uganda kaolins have a ples. In terms of trace element contents, the two higher sand component compared to those from kaolin deposits are very similar. The differences Latium (Italy). In contrast, the mineralogical com- between the trace element contents in the position diagram (Fig. 6) shows a closer similar- Buwambo and Migade deposits are so small that ity to high quality kaolins from Germany and the it seems that the deposits were derived from par- U.K. This comparison indicates that the Uganda ent rocks with similar chemical and mineralogi- kaolins have a potential for high quality indus- cal compositions. trial use. The effects of kaolinization and weathering are evident in the variable total REE contents of the DISCUSSION kaolins (Table 2). The REE abundances normalized to Singo granite and chondritic values are The mobility of chemical elements in natural shown in Figs. 4(a)Ð(d). The kaolins in general environments depends largely on their redox po- show uniform REE patterns, being more enriched tential and the pH of water. Chemical weathering, in LREE compared to the HREE. The kaolin sam- such as kaolinization of rocks, usually starts in a ples show a distinct negative Ce anomaly, which weakly acidic environment, but liberation of al- is a result of weathering (see below). kalis and alkali earths’ results in a rapid increase of the pH. Alkalis and alkali earths are quickly removed from the kaolinization environment, thus SUITABILITY FOR CERAMIC APPLICATIONS the last stages of kaolinization proceed in acid An assessment of possible ceramic applications environment. This change, from slightly acid to of Buwambo and Migade kaolins, was carried out strongly alkaline, and finally to acid conditions by comparing its composition with the kaolins is, of primary importance for the migration of sev- commercially marketed in Europe (cf., Ligas et eral chemical elements in the course of al., 1997). Figure 5 shows a comparison of the kaolinization. Progressive weathering and The Buwambo kaolin deposit in central Uganda 253 kaolinization of the granite lead to decomposition Leaching of these elements in Buwambo deposit of all minerals except quartz in the Buwambo de- proceeded at varying rates, depending on the re- posit. The decrease in the SiO2 content relative to sistance of the elements-bearing minerals to the Singo granite shows an advanced stage of weathering, kaolinization, and on physico-chemi- kaolinization for the Buwambo kaolinite, as the cal conditions (e.g., pH, Eh). Assuming that weath- solubility of quartz increases in an alkaline envi- ering proceeded under similar chemical conditions ronment (Garbarino et al., 1994). Successive de- in the Buwambo and Migade deposits, the differ- creases in the SiO2, MnO, MgO, P2O5, and more ence in the elements composition in the kaolin so in the Fe2O3, TiO2, CaO, Na2O, and K2O con- deposits resulted by kaolinization process. Stron- centrations shows their higher mobility. The re- tium is more concentrated than Ba in all samples. duction in these oxides is compensated by enrich- Strontium is mostly bound in plagioclase, where ment in Al2O3 and higher LOI. it replaces Ca. Barium has a behavior similar to The presence of TiO2, Fe2O3, and MnO has a that of K, and is mostly concentrated in K- negative effect on the quality of Buwambo kao- feldspars and micas. Both Ba and Sr are easily lin. In kaolins, Ti occurs in small amounts, mainly mobilized during weathering and are readily re- in the form of anatase, which in finely dispersed moved from the environment, but leaching of Sr form is difficult to determine by XRD (Fischer, starts much earlier and is more intense. Calcium 1984). However, during weathering and and K are almost completely removed by weath- kaolinization small amounts of Ti enter the ering, but significant amounts of Sr and Ba, which kaolinite structure (Murray and Keller, 1993) and have a close affinity to these elements, remain in it enhances the coloring effect of iron in kaolins the kaolin. Presumably Sr and Ba are partly (Fischer, 1984). In the Buwambo kaolin, iron oc- adsorbed by clay minerals or were added to a small curs in form of hydrated oxides adsorbed on small extent by downward percolating solutions. kaolin grains and in secondary iron minerals Potassium, Rb, and Cs are mainly hosted in formed in the course of weathering and micas and K-feldspar (Heier and Billings, 1970), kaolinization. In small amounts iron enters the thus alteration of these minerals will dominate the kaolinite structure and replace aluminum in the fractionation of these elements. Rubidium has a octahedral layer (Murray and Keller, 1993). The trend comparable with that of Nb and V and it is MnO content in Buwambo kaolin may be corre- less mobile than K, whose behavior it generally lated to biotite, hornblende, and, to some extent follows. This has been explained in that Rb, in in other iron-bearing minerals. During weather- contrast to K, is preferentially retained in weath- ing and kaolinization, Mn is easily oxidized to ered illite (Garrels and Christ, 1965), but Nb and Mn4+ and immobilized in the form of oxides, along V are less mobile (Middelburg et al., 1988). The with the rise in pH. In a high reducing environ- concentration of these elements in the Buwambo ment, Mn enters the structure of carbonates, in kaolin is controlled by the phyllosilicates. Feng which Mn2+ may replace Fe2+. However, in an acid and Kerrich (1990) noted that Cr, Co, Ni, Ti, and environment, Mn is leached during kaolinization. V behave similarly during magmatic processes, Similarly, trace element contents in both but cautioned that they may be fractionated dur- Migade and Buwambo kaolins decrease with in- ing weathering. The high Ni content in the kaolins creasing extent of weathering and kaolinization is related to its being easily mobilized during (Fig. 3) compared to the Singo granite. The higher weathering (cf., Turekian, 1978). Scandium has concentration of Ni in the kaolin samples may be been slightly concentrated in the kaolin during due to the mobilization of iron oxide. Trace ele- kaolinization compared to other trace elements. ments are liberated from primary structures dur- The tendency of Cr to be removed is probably re- ing granite weathering and kaolinization processes lated to Cr4+ oxidation, soluble as chromate ion and are subsequently removed from the rocks. (Marsh, 1991). Thorium and U behave differently 254 G. W. A. Nyakairu et al. during weathering, in that U, unlike Th, is chemi- LREE (Nesbitt, 1979), a basic pH environment is cally mobile as U6+ and is precipitated as insolu- also known to fractionate LREE-HREE, as the ble U4+ compound. Thorium and U are mainly HREE are preferentially retained in solution, accommodated in some heavy minerals, such as where they form soluble complexes (Cantrell and zircon, which are resistant to weathering. The low Byrne, 1987). trace elements contents in the Buwambo kaolin However, some trace elements and the LREEs samples in comparison to Singo granite can be are extremely concentrated in the kaolin samples. accounted by their clay mineralogy with notice- For example, BW-3 and BW-9 shows extraordi- able quantities of kaolinite, in addition to the fact narily high concentrations of Sr, Ba, Zr, Y, LREE, that these elements are preferentially fixed by illite and P2O5. The high concentrations of the LREEs as K2O reveals. However, the kaolinite percent- and P2O5 in these samples strongly suggest con- ages in kaolin samples constitute an additional trol by phosphate minerals, such as apatite and evidence of intensity of weathering and monazite, during weathering and kaolinization kaolinization extent. processes. The high Zr and Y contents are associ- The Singo granite-normalized REE patterns ated with heavy minerals such as zircon although (Fig. 4) show LREE enrichment, a slight positive the zircon content hardly explains the low abun- Eu and a negative Ce anomalies, and HREE de- dance of Hf. The high level of Sr in the samples is pletion in the Buwambo kaolin with the excep- due to the uptake of Sr in magnesite and arago- tion of BW-3 and BW-9 samples showing higher nite lattice. High Ba content is due to the pres- contents. Compared to Buwambo kaolin, Migade ence of feldspar and clay contents. However, their kaolin shows no Eu anomaly. The chondrite-nor- distributions are not only controlled by zircon, clay malized REE patterns of the kaolins are identical minerals, or phosphates, but a rather combination with LREE enrichment, a negative Ce anomaly of these minerals. The LREEs enrichments in the and HREE depletion with samples BW-3 and BW- Migade kaolin show that the Migade kaolin has 9 showing higher contents. However, Migade kao- undergone more weathering and has been lin shows more REE contents than Buwambo sam- kaolinitized to a greater extent. ples. The LREE enrichment reflects the results of extreme weathering while the slight positive Eu CONCLUSIONS anomaly is due to plagioclase or orthoclase presence in the kaolin samples. Weathering causes Kaolinite is the dominant mineral in the fractionation between LREE-HREE and between Buwambo and Migade kaolins, followed by Ce and other REE. The REE are released from quartz, with muscovite/illite as accessory miner- primary minerals and taken up by the secondary als. The chemical composition of the kaolin is phases during kaolinization. The REE are leached essentially SiO2 and Al2O3, with minor amounts by local meteoric waters from the upper levels of of TiO2, Fe2O3, MnO, MgO, K2O, and P2O5. The the profiles and concentrated in deeper zones, decrease in SiO2 content relative to the Singo gran- probably in the form of complexes, or by mechani- ite indicates kaolinization in alkaline environment, cal transport of secondary phases (clay minerals and Al2O3 has remained immobile. As TiO2, Fe2O3, and oxides) (cf., Prudencio et al., 1989). However, MnO, and MgO are known to negatively affect Ce has a different behavior, as it may be partly the kaolin color, the Buwambo and Migade kaolins oxidized to Ce4+, which is retained in the upper have almost a white color. The loss in MgO, CaO, levels of the soil profile relative to the other tri- Na2O, and K2O indicate the extent to which valent REE. This accounts for the observed nega- kaolinization of the Buwambo kaolin has under- tive Ce anomalies displayed in the Buwambo kao- gone. The kaolin samples from Buwambo are quite lin samples. Although kaolinite accommodates the pure and are virtually equal to the chemical com- REE (Prudencio et al., 1989), and particularly the position of pure kaolinite. From chemical and The Buwambo kaolin deposit in central Uganda 255 mineralogical composition, Buwambo and Migade from Sardinia (Italy) and genetic implications. Chem. kaolins are suitable for the production of earthen- Erde. 54, 213Ð233. ware, sanitary, white ware, and electrical insula- Garrels, R. M. and Christ, C. L. (1965) Solutions, Min- erals, and Equilibria. Freeman and Cooper, San Fran- tors production. cisco, California, 450 pp. Compared to average source rock abundances, Geological Survey, Uganda (1962) Geological map of e.g., the Singo granite, the Buwambo and Migade Kampala. Sheet NA 36-14. kaolins are depleted in all trace elements and only Harris, N. (1946) A note on kaolin supply near Bombo. Ni shows an enrichment. Assuming uniform Report Geological Survey and Mines, Uganda, 2 pp. Heier, K. S. and Billings, G. K. (1970) Rubidium. Hand- weathering conditions, Ni has been concentrated book of Geochemistry (Wedepohl, K. H., ed.), 37B1Ð by the kaolinization process. The Singo granite 37N1, Springer, Berlin. normalized REE patterns shows LREE enrich- Ligas, P., Uras, I., Dondi, M. and Marsigli, M. (1997) ment, with a slight positive Eu and a negative Ce Kaolinitic materials from Romana (north-west Sar- anomalies for the Buwambo kaolin. The chondrite dinia, Italy) and their ceramic properties. Appl. Clay normalized REE patterns, however, show little Sci. 12, 145Ð163. Marsh, J. S. (1991) REE fractionation and Ce anoma- change, except a slight LREE enrichment and a lies in weathered Karoo dolerite. Chem. Geol. 90, negative Cerium anomaly. The Migade kaolin has 184Ð194. undergone a higher degree of weathering and Middelburg, J. J., van der Weijden, C. H. and Woittiez, kaolinization than the Buwambo kaolin. J. R. W. 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