BY STAFF REPORTER
Marble has been mined in many parts of Ethiopia since ancient times and the deposits are inexhaustible. There is a wide range of colours and the quality is supreme. The quality is so good that Ethiopia has a significant export to the marble country par excellence of Italy. Numerous extensive deposits of marble are generally hill forming and occur throughout northern and western Ethiopia. The marble deposits are part of medium grade, meta sedimentary successions of the different Precambrian greenstone complexes. The marbles occur as strike parallel lenses (presumably reflecting fold patterns), intercalated with amphibolite, quartzite and schists. The meta sedimentary rocks are affected by granitic, mafic and ultramafic intrusions.
Most of the marble is white to grey, coarse-grained calcitic with calcite contents of 98per cent. All marble deposits are jointed and fractured. Locally the fracture systems are closely spaced but often joint systems occur with spacing between 2–3 m, making quarrying of large blocks feasible.
Most marble deposits contain billions of tons of high-quality marble. The Ethiopian marbles vary within a large colour spectrum. Pure white marble is common together with grey, greenish, pink and sky blue varieties.
During the last 20 years, the production and use of building stone has steadily increased worldwide, and today stone has reached a position as one of the world’s most important mineral resources. For many countries, export of stone has become a significant economic activity. For others, the recognition of local sources of building stone has secured a steady supply of cheap and durable construction materials for domestic purposes.
Almost any type of rock that can be shaped and dressed to blocks and slabs can be considered as a potential building stone source. Most commonly exploited are massive rocks such as granite and other igneous rocks, marble, limestone and sandstone, and slab rocks such as slate and flagstone. For local housing and other construction, soft and/or easily cleaved rocks are preferred, since they can be worked with simple tools. On the international market, however, the aesthetic properties of the rocks (colour and structure) are far more important, and generally the pricing of building stone depends on the exclusiveness of the rock. Rare colours such as blue, pure white, pure black, yellow and emerald green are considerable higher priced than the more ‘ordinary’ colours.
During the last decades, and especially during the 1990’s, systematic prospecting for building stone in Ethiopia has been carried out by both the EIGS and private companies, and a number of building stone deposits throughout the country have been put into production. This is reflected by the extensive use of Ethiopian stone in new buildings in the capital and other cities.
In the exploitation of massive stone (dimensional stone), large, commercial blocks are extracted in the quarry and transported to a processing plant for final shaping and finishing to slabs and tiles. The most homogeneous and attractive types of rocks may be exported to other countries as rough blocks. Generally, deposits suitable for exploitation of massive stone should be very homogeneous, both in colour and in structure. Furthermore, the occurrence of joints, veins and other factors that contribute to a reduction in the overall block yield, should be very small, except for extremely high- priced (rare) rocks. The most obvious resource potential within the investigated regions in Ethiopia lies in the huge marble formations in the western part of the country, and in different varieties of granites and gneiss, limestone and serpentinite/soapstone.
Market evaluation of such deposits is difficult. Firstly, there are no objective price lists available on the international market, and prices can vary greatly due to changes of fashion and personal taste of the customers. Secondly, very few producers actually go public with their prices, and a picture of the market situation implies a need for experience among the professionals involved in the evaluation of stone deposits. Furthermore, the market for rough blocks is significantly different in comparison with finished products. In the former case, the customers are stone-producing companies around the world, whilst the customers of finished products are end-users (architects and constructors). The notable difference in raw material prices for different rock types may not be as obvious for finished products. Generally, however, the price difference between highly attractive and less attractive rock types is much higher for raw blocks than for finished products. In this article, we have used published and nonpublished price lists for raw blocks of comparable types of stone as a tool in market evaluation. Due to the subjective character of such information, price indications given below must be regarded as indicative only.
In Ethiopia, the use and domestic production of cut-to-size stone have increased during the last decades, contributing positively in the development of a high-quality, domestic industry. For the major part, dimension stone used in Ethiopia is derived from domestic sources. Although limestone and marble have been produced for a long time, the production and use of siliceous rocks has only recently started, and it is within that sector we would expect the most significant growth in the country’s dimension-stone production in the years to come.
The most interesting deposits of marble are found in the western part of Wellega (Daleti) and Gojam (Mora, Bulen, Mankush and Baruda). The area is quite remote, and transport distances to Addis Ababa vary between 550 and 800 km, for the most part along non-paved roads. However, at the present time, transport costs are still low enough for the marble-producing companies to find the production profitable.
Most of the marble deposits found in these areas form an elevated morphology, others have a flat, though well exposed, morphology. Predominantly, the marbles are cal-citic, but white to grey dolomite occur as layers within the calcite marbles or as bordering units. The latter has, until now, not been of significant interest as building stone.
Several types of commercial marble occur in the area (Malis & Dejene 1983, Heldal et al. 1997). These include fine to medium-grained, graphitic grey marble with white bands, medium- to coarse-grained white marble with grey bands (Fig. 2) and several subordinate types such as pink, silicate rich marble, pure white, fine-grained marble and sky blue to green marble. The latter two types seem, both from field observations and comparison with description of similar deposits in Norway, to be connected to contact-metamorphic aureoles surrounding gabbro intrusions. Grain size varies from fine- to coarse-grained, and these variations are believed to be the result of local variations in metamorphic conditions, probably caused by heating related to syntectonic intrusions, and grain-size reduction during later deformation episodes, especially in high-strain zones. In the latter case, the marbles have often a schistose, ‘slabby’ appearance, and are not suited for production of squared blocks. Fold patterns may be very complex within some of the marble deposits, especially where competent layers of dolomite are interbanded with less competent calcite marble. An example from one of the complexly folded Gojam deposits is shown in Fig. 3. Measurements of fold axes further show that the folds are non-cylindrical, demanding care in estimating subsurface marble reserves.
Pre-syn- and post-tectonic dykes and veins of acidic and basic composition are common in several of the marble deposits (Fig. 4). Particularly, the foliation-parallel, folded dykes cause problems for the extraction of marble, since they infiltrate a large part of the rock mass and are difficult to predict.
Thus, in such areas, the block yield and the possibilities of using sophisticated quarrying technology, such as diamond-wire sawing, are reduced. Furthermore, closely spaced joints cause similar problems in many cases. However, both the existing quarrying activity by several companies and recent investigations by the EIGS indicate a fairly good potential for increasing the production of low-to medium priced, grey and white marble from the area. Furthermore, investigations suggest a possible development of highly exclusive types of marble, such as pure white and sky blue varieties. The latter two may obtain very high prices on the international market, but the deposits discovered to date are small and inhomogeneous.
THE ETHIOPIAN HERALD JULY 16/2021
