Regional geochemical soil surveys in West Africa have led to the discovery of many mineral deposits. Some of the major mineral deposits found in Mali, Burkina Faso and Niger with geochemistry include Syama (gold), Loulo (gold), Sadiola (gold), Perkoa (zinc-silver-lead) and Koma Bangou (gold).
The Syama deposit in southern Mali, which is being exploited by BHP Minerals, was first indicated in 1980 by the United Nations Development Program (UNDP — a technical assistance program offered only to government departments or government-owned agencies), following a regional soil geochemical survey (1,000 x 200 metres) which showed an anomaly in the vicinity of old orpailleurs workings.
This program was followed by more detailed soil sampling (100 x 40 metres) which defined an anomalous zone measuring some 1000 x 300 metres enclosing gold values ranging from 1 gram gold per tonne to 10 grams.
The anomaly was centered over a gold reserve later (1989) defined by BHP to contain 3 million tonnes of oxide ore grading 3.2 grams and 21 million tonnes of sulphide ore grading 4.02 grams.
The Loulo deposit was found in western Mali by the BRGM-Mali Gold Exploration Syndicate in 1980 by prospecting geochemical gold soil anomalies first indicated in a regional survey (1,600 x 500 metres) and then defined by more detailed (200 x 100 metres) geochemical soil surveys.
The deposit is reported to contain 2.3 million tonnes of oxide gold bearing ground grading 3.7 grams and 2.3 million tonnes of sulphide material grading 4.7 grams.
The Sadiola gold deposit was discovered in western Mali as a result of a regional geochemical soil survey (1,000 x 200 metres) sponsored by the European Development Fund in 1987. Detailed soil sampling (100 x 80 metres and 100 x 20 metres) over anomalous areas followed in 1988.
Subsequent work by AGEM Ltd. and Anglo American has defined a probable reserve of 49.3 million tonnes oxide and mixed oxide-sulphide ore grading 2.57 grams. Currently, Sadiola is being readied for production by Anglo American.
The Perkoa volcanogenic massive sulphide deposit was found in 1982 following a regional soil-sampling (400 x 400 meters) program carried out under the auspices of the UNDP in western Burkina Faso. The initial regional survey showed two anomalous zinc values of 214 parts per million (ppm) and 286 ppm (regional background of 20-30 ppm zinc) over the sub-cropping mineralized zone (Figure 1a).
Subsequently, semi-detailed (250 x 250 metres) and detailed (50 x 25 metres) geochemical soil sampling confirmed the existence of an area of 550 x 250 metres anomalous in zinc (>200 ppm, see Figure 1b) with coincident anomalous lead and silver values (Lewis et al 1989 and Salamis 1994).
Initial drilling in 1982 intersected a massive sulphide zone in which has been defined a minable reserve of 5.5 million tonnes grading 18.2% zinc and 50 grams gold. The feasibility of mining the deposit is currently being evaluated by Perkoa Mineral S.A. (35% state and 65% Boliden). Large portions of the greenstone belts in Mali and Burkina Faso have been covered by regional geochemical soil surveys sponsored by the UNDP and other aid agencies. This work has resulted in the discovery of many gold targets which are now the focus of exploration by various major and junior mining companies.
In the Liptako area of Niger, geochemical soil and termitary surveys sponsored by the Canadian International Development Agency in 1985 have demonstrated the effectiveness of these surveys in defining gold mineralization in the Tera-Gassa and Sirba greenstone belts. The soil and termitary gold geochemistry was an important guide in helping to define a significant auriferous occurrence at Koma Bangou. Here, a possible geological reserve of some 10 million tonnes grading 2.47 grams reportedly has been defined in quartz veins and stockworks hosted in granodiorite and volcano-sedimentary rocks. The deposit is being evaluated by Etruscan Enterprises (VSE).
The termitary sampling was more effective than near-surface (20-30 cm depth) soil sampling in areas covered by allochthonous material, which may consist of alluvial sands and gravel, eolian sands and lacustrine silts and clays deposited in low areas from intermittent ponds and lakes formed during the wet season.
Throughout West Africa it has been the practice to take soils at a predetermined depth, usually at about 30 cm. This practice can result in the sampling of a variety of regimes, some of which may be residual and others allochthonous. Residual regimes would include terrains underlain by laterite or saprolite.
One can expect fluctuations in the metal content of soils depending on which regime is sampled. Element levels can be significantly different from laterite versus saprolite. However, the biggest difficulty arises when one mixes geochemical results for soils from a residual regime with surface materials from what might be termed an allochthonous or depositional regime. These would be environments where the laterite or saprolite has been covered by colluvium, alluvium, eolian sands or lacustrine deposits. Sampling of such materials will define transported anomalies or negative results over mineralized systems. In such environments it is necessary to sample at depth using hand dug-pits, augers or drill holes.
The most useful surface material to sample in these allochthonous regimes are termite mounds. Termites tunnel to the water table and build their mounds from material excavated at depth. They have been known to bring material from depths of up to 70 metres. Like the gold mineralization, many aquifers in crystalline rocks are structurally controlled. As a result, the termites physically transport gold grains to the surface from auriferous structures which can easily be found by panning the material in the termite mound. Geochemical analyses of material from termitaria reflect underlying mineralization and are independent of any transported overburden overlying the laterite/saprolite horizons.
Therefore, one may conclude that large, near-surface, supergene gold deposits as well as base metal deposits in laterite terrains can be found in West Africa by the diligent application of regional and detailed scale geochemical soil sampling programs.
For the technique to be most effective, a good understanding of the regolith situation and the position within the profile from which the soil sample is derived is required. Proper terrain analysis prior to carrying out geochemical soil surveys will help control the sampling program. (References:
(1) Lewis, M., Mage, R., Dakio, J., and Ouedraogo, J. 1989: L’Application des Mthodes Gophysiques et Gochimiques pour la Recherche des Gisements de Sulfures Massifs de Type Perkoa. Proceedings of Exploration’87 OGS Sp. Vol. 3, p837-854; edited by G.D. Garland.
(2) Salamis, C. 1994: Perkoa, Burkina Faso, West Africa: A Massive Sulphide Zinc-Silver Deposit. Proceedings of a short course presented by the PDAC, March 5 and 6, 1994, p5-1 to 5-45.)
— Christopher Gleeson, PhD, P.Eng., is a resident of Iroquois, Ont.
Be the first to comment on "MINING IN AFRICA SPECIAL — Examples of geochemical"