AFRICA — Soil geochemistry spearheads Robex gold search — Surveys in Mali show potential for Sadiola-style mineralization

Much as pioneering mineral exploration in northern Canada relied on the maps drafted by government surveys, the efforts of development agencies to provide a geological data base for West Africa have begun to pay off.

In the sub-Saharan grasslands of western Mali, Robex Resources (RBX-M) has used earlier geochemical surveys funded by the European Community to select areas with potential for gold mineralization, and has followed up with some shrewd geochemical work of its own.

Modern exploration in western Mali has its roots in the late 1980s, when the European Community funded a reconnaissance geochemical survey by the German firm Klockner. The survey covered large areas underlain by early Proterozoic-age Birimian greenstones, the same belts that host the large gold mines of the Obuasi district in Ghana.

The survey’s first success was the delineation of the Sadiola gold deposit, now being operated by Anglo American, and held by Anglo, Iamgold (IMG-T), the government of Mali and the World Bank’s International Finance Corporation.

Sadiola, about 70 km by road from Kayes, and on the Bamako-Dakar railway, now produces about 400,000 oz. annually.

The survey also revealed a zone of soils with high gold contents in the Diangounte area, about 40 km southwest of Sadiola. But whereas the gold deposit at Sadiola was chiefly developed in carbonate rocks of the Kofi formation, the soils at Diangounte formed over a terrane of Birimian volcanic rocks (the Saboussire formation) intruded by granitic rocks. Both, though, were centred on large regional faults striking roughly north.

Klockner’s geochemical anomaly at Diangounte, at 80 sq. km, was similar in size to the Sadiola anomaly; both anomalies showed peak gold concentrations around 1 gram per tonne. A further similarity was the strong structural element to the gold zones — those on the Diangounte anomaly shared a common north-northeast trend.

Robex Resources acquired a 51% interest in concessions at Diangounte and Kata (also called Nienngue) in August 1996, under an agreement with a Malian company, N’Gary Transport. The companies have since reached another agreement, under which Robex is earning an 85% interest by making cash payments totalling $500,000 and fulfiling a $3-million work commitment over the term of the mining concession. Another US$1.5 million will buy a further 5% interest.

Robex concentrated its early efforts at Diangounte around the La Corne placer in the southwest corner of the property, where small-scale craft mining was already going on. The showing derives its name (“the horn”) from early production figures: a placer pit could produce a cattle-horn full of gold each day.

The placer was at the northern end of the Dialaki corridor, a string of gold showings 3 km long, 500 metres wide, and parallel to the anomalous zones in the geochemical survey. Old trenches mark its south end, and grab samples from outcrops and old workings along the corridor have yielded grades up to 400 grams gold per tonne.

In a large geochemical program, Robex collected 1,900 surface soil samples, 400 deeper auger-drilled soil samples, 1,500 rock samples and 1,300 trench samples.

Because coarse free gold was common in the laterite and saprolite soils, Robex attempted to pre-concentrate samples using a small Knelson concentrator. The Knelson consumed as much as 400 litres of water per sample, which placed a heavy burden on the local water supply. This forced Robex to stop operating the concentrator in February 1997.

During the time the concentrator was in operation, some gold was failing to sink in the concentrator water, and being washed away. Local craft miners insisted that fine gold frequently floated on clean water; their remedy was either to add tobacco juice (courtesy of craft miners with a chewing habit) or wood ash; Robex staff chose to standardize their process with wood ash added to the water.

A reconnaissance soil-geochemistry survey revealed five zones where soils routinely held more than 50 parts per billion of gold (0.05 gram per tonne).

All the anomalous zones coincide with northeast-striking shear structures and with southeast-striking cross fractures. Some of the geochemical values exceeded 1 gram gold per tonne, and the peak value in the Dialaki corridor was 2.3 grams per tonne.

Trenches and pits along the Dialaki corridor all showed traces of gold with four trenches yielding grades of 1.6 to 6 grams gold per tonne over standard 1-metre intervals, and four pits returning bulk values of 5.3, 5.8, 10.6 and 21.6 grams gold per tonne. As is typical of West African gold showings, grade increases in the profoundly weathered saprolite material that overlies the bedrock.

Future work will include further auger drilling, pitting and trenching, plus magnetic and induced-polarization (IP) surveys over the geochemical anomalies. At La Corne, a systematic auger drilling program has been recommended, to be followed ultimately by diamond drilling.

Robex also sampled an ultramafic body near the northern boundary of the property, which proved to carry 0.05% vanadium, 0.1% copper, 0.01% chromium and 0.14 gram gold. The samples come from the leached zone, where metal values are usually depleted.

Further work is planned on the ultramafic body, which underlies about a fifth of the property.

At the Kata (Nienngue) property, about 35 km east of Diangounte, Robex has finished another reconnaissance-level geochemical survey, covering a 57 sq.

km grid on 500-metre-by-200-metre intervals. Here, the company is using a 5-kg soil sample, which is split into three parts: two 500-gram soil samples, one sent for chemical analysis and the other retained as a backup, and a 4-kg sample washed in a calabeza gourd, producing a concentrate for gold-grain counting.

The soils had gold contents as high as 1 gram, and washed concentrates from several locations yielded counts of 20 to 50 gold grains. Two “corridors” of gold showings have been defined — the 8-km-long CK-1 corridor and the 5.5-km-long CK-2. Both include areas with artisanal placer workings.

The 58-sq.-km property at Kata is about 40 km southeast of Sadiola and is underlain by strongly weathered sedimentary rocks of the Kofi formation. One granitic intrusion is known on the eastern boundary of the concession. The sediments include some nodules of carbonate rock that appear to consolidate with depth, indicating the possibility that the sediments may have interbedded limestone layers.

At Sadiola, the gold mineralization is mainly developed in carbonate rocks adjacent to a major north-south fault. Accordingly, Robex is pursuing a “Sadiola” model at Kata, and will be following up on the geochemical results with trenching. Ground magnetic surveys and IP surveys are also scheduled.