MINING IN AFRICA SPECIAL — Diamond deposits on W. African,

West Africa’s diamond potential was among the topics discussed at a recent symposium in Toronto titled “The geology and mineral deposits of the Guianan-West African Shield.”

Sponsored by the University of Toronto student chapter of the Society of Economic Geologists, the symposium included sessions which attracted the interest of companies exploring for diamonds in Africa.

Among these was a presentation by Henry Meyer of Purdue University’s department of earth and atmospheric sciences, who compared diamond deposits on the West African and Guiana (South America) Shields.

Meyer noted that for most of the Precambrian, the Amazonian and West African cratons were a single unit and in relatively close proximity. Several of the thermotectonic events recorded in the rocks of the Guiana Shield can be correlated with ones that affected the West African craton.

Age-dating studies have shown, however, that apart from the diamond-bearing rocks in Ivory Coast and Ghana, all others in West Africa were formed after the separation of the two continents.

Meyer said most diamonds in the Guiana Shield are recovered from alluvial sources of limited extent and that, to date, no major producer has exploited the deposits economically. As well, no major exploration has been undertaken over the Guiana Shield in spite of some major groups having looked at localized areas.

In contrast, Meyer noted, West Africa is better explored and exploited, having large-scale economic operations for diamonds since the 1930s, in both Sierra Leone and Ghana (and, more recently, in Guinea).

But in spite of the known occurrences of kimberlite and lamproite, the major successes in West Africa have been in mining the alluvial diamond deposits and not the primary sources.

One of the main reasons for this, Meyer said, is the small, and often stockwork, nature of the kimberlites. However, several large diamonds (more than 100 carats) have been recovered in Sierra Leone and Guinea, and such stones often enhance the economics from marginally economic to profitable.