Journal of African Earth Sciences 35 (2002) 123–134 www.elsevier.com/locate/jafrearsci Mineralogical, geochemical, and sedimentological characteristics of clay deposits from central Uganda and their applications George W.A. Nyakairu a,1, Hans Kurzweil b, Christian Koeberl a,* a Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria b Institute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria Accepted 10 October 2001 Abstract In Uganda, Precambrian rocks have undergone extensive weathering and erosion, and are locally altered to form considerable clay deposits. We have studied the geochemical, mineralogical, and sedimentological characteristics of clay deposits from central Uganda to determine their composition, source rocks, deposition, and possible use in local industry. Samples were collected from the Kajjansi, Kitiko, Masooli, and Ntawo deposits (near Kampala), all of which are currently used for both industrial and tra- ditional brick, tile, and pottery manufacture. The deposits are widely scattered individual basins, with clays deposited under la- custrine and alluvial environmental conditions, and were all found to belong to the sedimentary group. The clays are composed of silt–sand fractions and predominantly consist of kaolinite and have a relatively high Fe2O3 content. The studied deposits are chemically homogeneous, except for the samples richer in sand fraction, which have higher SiO2 and K2O values. The chemistry of the studied samples, compared to European clays, shows that they need elaborate treatment to render them suitable for ceramics production. An analysis of the chemical and mineralogical composition of the clays has demonstrated that, taken as a whole, they possess characteristics satisfactory for brick production. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Clay deposits; Precambrian rocks; Uganda 1. Introduction the ever-increasing market demand for the construction materials needed. Traditional methods of production, Clays occur widely in many parts of Uganda. Besides which do not take account of the chemical and miner- their geological interest, they are of importance for local alogical characteristics, are still practiced. In the tradi- industry. They have been used to produce rather poor- tional method of brick production, rawclay material is quality bricks, tiles and pottery by primitive methods for mixed with water and covered for about a week. The several years. Scattered clay pits and brick kilns along paste is placed in a wooden mould as shown in Fig. 2(a) the roadsides document the uncontrolled and low-tech- and (b). The bricks are spread and covered with cut nology exploitation of the Uganda clay occurrences. grass until they are dry. However, during the rainy Apart from artisan brick producers, there are organized season, plastic sheets are used to cover the bricks (Fig. clay works, such as Uganda Clays and Pan African Clay 2(c)). The bricks are fired in field kilns, which consist of Products at Kajjansi, and Allied Clays at Masooli along a large pile of unfired bricks with tunnels in the bottom the Gayaza road, which supply the construction indus- of the pile (Fig. 2(d)). The pile is cemented with clay and try in Kampala and surroundings. Starting in 1986, contains 10,000–15,000 bricks. A wood fire is built in the there has been an increase in construction activity in tunnel and kept burning for 4–6 days and the tunnels are Kampala. The above-mentioned industries cannot meet then closed with unfired bricks and also cemented with clay. The hot exhaust from the wood fire flows through the pile, and heats the center of the pile enough to fire * Corresponding author. Tel.: +43-1-4277-53110; fax: +43-1-4277- the bricks in the core of the pile. The pile is then allowed 9531. E-mail address: [email protected] (C. Koeberl). to cool and dismantled. 1 Current address: Department of Chemistry, Makerere University, Fewstudies have been made of the clays used in the P.O. Box 7062, Kampala, Uganda. brickworks or of raw materials used for pottery in 0899-5362/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S0899-5362(01)00077-X 124 G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134 Fig. 1. Generalized geological map of the study and surrounding areas extracted from the geology map of Kampala sheet NA 36-14 (Geological Survey of Uganda, 1962). The inset is a map of Uganda. G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134 125 Fig. 2. Photographs showing the traditional brick production methods used in Uganda. (a) After a heap of raw clay material is covered for a week, the paste is placed into a mould. (b) Mould with clay paste ready for drying. (c) Two moulds used to make bricks, spreading of bricks for drying, cut grass for covering the bricks and also the plastic sheets used in the rainy season in the background, and finished bricks for kiln construction. (d) Partly finished typical brick kiln under construction, with tunnels in which wood fires are built to fire the bricks. Uganda. Harris (1946) and Kagobya (1950) studied the 2. Geology clay deposit at Ntawo, 25 km from Kampala on the Jinja road (Fig. 1). It was reported that Ntawo clay The study areas are indicated on Fig. 1. The areas are exhibited marked shrinkage and cracked on firing, and mostly underlain by Precambrian rocks that include the quality of product was inferior when it was evalu- sedimentary and metasedimentary lithologies, which ated for pottery production. McGill (1965) studied the comprise fine-grained sandstones, slates, phyllites, and nature and distribution of clays from several occur- schists. The more highly metamorphosed rocks include rences around Kampala, and determined their plasticity quartzites, muscovite–biotite gneisses, and subordinate with a view to establish a fine ceramics industry. Tu- schist, which may locally contain cordierite. The above humwire et al. (1995) measured the physical properties rocks, together with amphibolites and epidosites, are and discussed the geology of the Kajjansi and Kitiko part of the Buganda series, which make up a wider deposits located 13 km from Kampala on the Kampala– Buganda–Toro system with the Toro series of Western Entebbe road. A study of some clay samples from Uganda (Schlueter, 1997). In deeply weathered areas, various deposits in Uganda indicated that they are me- parts of the basement are exposed in the form of un- dium-quality kaolinitic–illitic clays (Nyakairu and Ka- differentiated gneisses and late granites, as well as mig- ahwa, 1998, and references therein). There is ample matized and remobilized parts of the Buganda series. demand for quality bricks and other clay products, and, These rocks are overlain in places by swamp deposits, thus, the present study evaluates the mineralogical and alluvium, and lacustrine deposits. The underlying gneis- chemical characteristics of the rawmaterial used, not sic and granitoid rocks of the Precambrian basement only in individual brick and tile works, but also by the have been extensively weathered and transported to traditional potters. This will help to give a better un- produce clays. Some of these clays are weathering derstanding of the clay materials, as well as of their products of schists and amphibolites, and of basic rocks geochemistry and source rocks. of the younger Buganda series. According to McGill 126 G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134 (1965), the clays can generally be classified as sedimen- Aliquots of 20–30 g of each dried sample were pow- tary and transported alluvial clays. It is from these clays dered in an agate mill. The chemical analyses (major that the samples used in this study were obtained. elements) were performed on powdered samples using Clays derived from gneissic and granitoid rocks of the X-ray fluorescence (XRF) spectrometry at the Univer- basement are leached and enriched in quartz. These sity of the Witwatersrand, Johannesburg, South Africa. clays are thought to have formed by leaching of the For details on procedures, precision, and accuracy, see decomposed bedrock, and are normally separated from Reimold et al. (1994). Mineralogical analysis was per- the bedrock by a layer of large quartzite pebbles (Harris, formed at the Institute of Petrology, University of Vi- 1946). The clays occur as surficial layers with a general enna, Austria, on bulk rock powders using a powder thickness varying between 2 and 5 m (cf. Kaliisa, 1983). X-ray diffractometer (Philips PW 3710) operated at The main features of the studied clays are their lowwet- 45 kV/35 mA using Ni-filtered CuKa radiation, with to-dry shrinkage, refractory nature, and an extremely automatic slit and on-line computer control. The sam- high plasticity, which is attributed to high kaolinite ples were scanned from 2° to 40° 2h. Mineral identifi- content. Due to their high plasticity, these clays can be cation on the diffractograms was processed using Philips classified as ball clays, which makes them suitable for PC-APD software, version 3.5B. The quantitative min- pottery, earthenware, and binders in refractory material eralogical composition was evaluated using the norma- production. In some areas, other clay occurs together tive calculation method of Fabbri et al. (1986). Total with the clays derived from gneissic and granitoid rocks carbon was determined with a LECO elemental analyzer of the basement in the same deposits. The bulk of these (Multiphase carbon determinator, RC-412) at the In- other clays are essentially micaceous schist derivatives, stitute of Petrology, University of Vienna, Austria. which gives them a yellowish-brown color, as opposed to the dull gray appearance of the clays derived from the 3.2. Grain size analysis results gneissic and granitoid rocks of the basement.