NOTE the Buwambo Kaolin Deposit in Central Uganda: Mineralogical And
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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 ele- ment 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 minera- logical 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 0.5–1.5-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. 248A. Nyakairu G.W. Table 1. Chemical and semi-quantitative mineralogical compositions of kaolins from the Buwambo and Migade deposits, central Uganda. The mineralogical composition calculations were performed from XRD spectra using a modified method after Schultz (1964). et al . Major element data in wt.%; mineralogical composition in wt.%; all Fe reported as Fe2O3; LOI = loss on ignition; data for U.K. (England) kaolin from Murray and Keller (1993); pure kaolin data from Newman and Brown (1987). The Buwambo kaolin deposit in central Uganda 249 flakes. Quartz also occurs in veins, 2–10 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 74–93 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 ele- ment 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.