PRELIMINARY FIELD RESULTS OF THE WEATHERING BEHAVIOUR OF THE CAVE HILL CARBONATITE, TORORO, EASTERN UGANDA

Gunnar Ries*, Inken Passe* & Andreas Schumann**

*University of Hamburg GPIuM, presently University of Dar es Salaam, Geology Dept., Box 35052, Dar es Salaam, Tanzania, e-mail: gunnar_ries@gmx.de, inken-passe@gmx.de
**Makerere University, Dept. of Geology, P.O. Box 7062, Kampala, Uganda, e-mail: eagn@infocom.co.ug

Abstract

This paper deals with the weathering residues of Oligocene carbonatites under warm-humid climatic conditions. Probably since the Late Cenozoic, laterites have been forming, deposited and preserved in fractures (karst caves) of the carbonatitic parent rock . Several stages of secondary calcite formation have been identified, corresponding to different phases of enhanced weathering periods. Some of the lateritic fracture fillings contain fossil gastropods.

Introduction

Rising 1482 m above sea level and nearly 300 m above the surrounding plain, Tororo Rock forms a prominent landmark in the south-eastern part of Uganda (Fig. 1), just a few kilometres from the Kenyan border. The Tororo Rock itself and the lower hills to the south, Reservoir Hill and Cave Hill, consists of mainly sövitic carbonatite. Williams (1952) describes them as a series of separately intruded collars and ring structures. Bishop et al. (1969), King et al. (1972) and Cahen et al. (1984) gave a summary on age determinations of the carbonatites of Eastern Uganda with K-Ar ages of 32 +/- 1,3 Ma for the Tororo Rock. Within the region the carbonatites are of economic importance and have been mined at Limekiln Hill for cement production. Although the carbonatite of Cave Hill has according to Tiberindwa (2000) a higher CaO content than the neighbouring Limekiln Hill, it also has a higher content of phosphate (> 5%), and is therefore not so suitable for the production of cement. Still the carbonatites are used by small scale miners for the production of aggregates and lime.

Figure 1. Map showing the location and geology of Tororo

Field Results

The weathering cover on top of the hills and along the slopes is generally thin (a metre or less) opposite to the preferentially thicker, mostly allochthonous regoliths in the valleys.

Cave Hill got its name because of the caves. They have been formed due to differences in the parent rock petrography. The fine grained carbonatite which contains layers of mafic minerals, responds much easier to weathering than the surrounding coarser grained, more homogeneous variety. Only where secondary calcite coatings have covered and hardened the finer grained carbonatite has it remained prominent. At Cave Hill up to six generations of calcite formation have been observed. The first and oldest is the calcite of the carbonatite itself, followed by hydrothermally precipitated calcite in veins. In some of these veins a third generation of calcite is found, formed by the precipitation of calcite from weathering solutions. In the field, the hydrothermal calcite is not easy to distinguish from the travertine formed by the weathering solutions. One criterion for distinguishing the travertine from hydrothermal calcite may be rhythmic layering of the calcite with laterite and the size and clarity of the crystals. The hydrothermal calcite is often white to colourless with well developed crystals whereas the travertine is mostly yellowish and milky with layers of laterite and shows smaller, less developed sub- to anhedral shapes. A fourth generation of calcite cemented the lateritic filling of fractures and a fifth is represented by very small calcite crystals in the cemented laterites. It ought to be noted that the cementation status is not uniform. There are non to very weakly cemented laterites (soft to moderately hard) and intensively cemented laterites (very hard). The different cementation status of the lateritic fracture fillings might itself indicate several generations of preferred calcite dissolution and precipitation. The sixth generation of calcite is represented by very recent thin travertine coatings found on laterites and on the parent rocks. These travertine coatings show sometimes well distinguished dripstones and even small stalactites.

One of the key localities is exposed on top of Cave Hill. In up to 3 m deep and 2 m wide karst caves - which developed along fractures - three generations of laterite and five generations of calcite can be distinguished (Fig.2). The different periods of laterite deposition may have been coinciding with repeated opening of the fracture.

Figure 2. Lateritic fracture filling on top of Cave Hill. 1 Carbonatite, 2 medium cemented laterite with rootlets and fossils, 3 cemented laterite, 4 uncemented laterite, 5 travertine coating and dripstones. The size of the bar is two meters.

In the medium intensively cemented laterite filling small rootlets are found, now hardened by iron crusts. This laterite is fossil bearing (Fig.3). According to a preliminary determination the fossil gastropod could be a Limicolaria of Late Pleistocene age (Pickford, 2001; pers. communication). The filling of the gastropod shell is the same as the matrix surrounding it. In addition to fossils the lateritic filling also contains subangular clasts of the carbonatite. This may indicate high rainfall conditions during the deposition of the lateritic filling and a rapid filling of the fracture.

Some fractures also contain manganese-iron crusts, often related to the weathering of mafic dykes (glimmerites). Iron-manganese precipitation sometimes also occurs in the form of small pisoliths, cemented by calcite.

Figure 3. Fossil gastropode from Cave Hill

Further research

The samples collected during the field campaign are now subject to further mineralogical and geochemical studies using appropriate analytical methods (e.g. XRF, XRD, SEM, EDAX). Apart from trying to look at parameters or characteristics which might help to differentiate the different calcite generations, element migrations will be investigated (dissolution of primary formed minerals, element migration and precipitation in secondary phases). This will include the initial weathering of the carbonatite and associated alkaline rock types as well as the concentration of iron and/or manganese in secondary precipitates and possibly other accompanying elements such as REE, which are known to be hosted in them (Schumann, 1996; Valeton et al., 1997, Mutakyahwa et al., 1999). The fossil gastropods (once determined without doubt) should help to date the time when particular karst caves have been filled.

Acknowledgements

The authors would like to thank the staff members and officials of Tororo Cement Factory, who gave us the permission to work in their concession areas and for their logistical support as well as Dr. John Tiberindwa (Dept. of Geology, Makerere University) who introduced us to the parent rock petrology of the Tororo carbonatites. Dr. Immaculate Ssemmanda (Dept. of Geology, Makerere University) is thanked for having us introduced to Prof. Martin Pickford (Laboratoire de Paleontologie du Museum National d'Historie Naturelle, Paris, France), who kindly had a look at the snail photographs (which we had sent to him via e-mail) and giving us a first opinion of the gastropod species which we had found in the cemented laterite. The German Academic Exchange Service (DAAD) is thanked for granting Mr. Ries a Ph.D.-scholarship to carry out his research in East Africa.

References

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Paper first published in GSU Newsletter 1 (1) April 2001, Kampala, Uganda