4. The 'Hombori marble'

Geologically, Hombori belongs to a domain where quartzites and greywackes are intercalated with various rocks including schists, dolomites, dolerites and radiolarites. The quarries are located in the Dimamou area, 30km north-west of Hombori (Fig. 1). Craftsmen went there on foot or on donkey-back.

Figure 1: Geographical map of the study area (from Microsoft Map Point)

The Dimamou quarries belong to the upper dolomite sequence of Irma, of Precambrian age (Défossez 1962). This sequence has been recognised in the Boundouyerou borehole, between Kounia and Koire (Fig. 2). It is located below a level of upper continental slate clay, at a mean depth of 70m. The Irma dolomite sequence includes several levels of schists grading down, upon contact with the granite, to glauconitic sandstones. Further away from this contact, the sediments consist mainly of calcareous rocks deposited in a subsiding environment. To the north, a lateral rock change occurs with the deposition of the Ydouban sequence below Mount Hombori. This sequence, hardly metamorphosed, consists mainly of reddish schists with some intercalated greywackes and tuffs. However, one still observes rare dolomitic levels from the Irma sequence, such as those outcropping in Dimamou (Défossez 1962).

Figure 2: Geological section along the Hombori meridian (from Defossez 1962)

The Dimamou limestone, locally called 'Hombori marble', forms a 1.5 to 2.5km large slice, intercalated within the Ydouban schists. Locally, thin levels of clayey sandstones are observed within the calcareous beds. In thin-section, it is a marble made of calcite phenocrysts with occasional quartz crystals. According to the available chemical data, it is a dolomitic limestone. This marble, which is exceptionally thick, marks the lateral transition from the Irma to the Ydouban sequence (Défossez 1962, 35). The stone appears in the form of outcrops, which extend over many kilometres and are orientated north-south.

Figure 3: Location of quarries (drawing by Y. Pailler)

Three quarry sites have been exploited in the Dimamou area: Beligousso, Dimam Berri and Nokema (Fig. 3). At Beligousso, a series of quarry sites extended in a straight line for many hundreds of metres; Tontoni worked there for 15 years with Babou and Mamoudou Salia. Further on in this direction there are the Dimam Berry quarry sites, where a reddish stone is found in places, much sought after on account of its rarity. Finally, the Nokema quarry sites (Fig, 4), where a darker stone appreciated by the Tuareg is found, are the largest, extending aver many hectares.

Figure 4: Photograph of outcrops at Nokema (picture by A. Garin Carmagnani, Y. Pailler)

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