STAGES IN THE TROPICAL WEATHERING OF KIMBERLITE

A B S T R A CT : Numerous exposures of highly altered kimberlites in exploratory trenches and pits have afforded an opportunity of studying the very great changes which take place chemically and mineralogically under conditions of tropical weathering in Sierra Leone. Diamondiferous gravels in the Kono and Kenema District of Sierra Leone, West Africa, have been worked continuously by Sierra Leone Selection Trust Limited since 1933 and constitute one of the world's major sources of gem and industrial diamonds: see map, Fig. 1. The only previous description of the Sierra Leone kimberlites has been given by Grantham & Allen (1960) whose paper contains a concise account of the original discovery of kimberlite in 1948, describes its petrography and that of its xenoliths in detail with photographs of thin sections, and includes a map of the Yengema-Koidu area showing the distribution of the dykes as then known. During the past 4 years considerable further work has been achieved in tracing these dykes along strike by excavating lines of pits to bedrock and panning the overlying residual/alluvial gravels for 'kimberlitic minerals' (mainly ilmenite with subordinate pyrope garnet); a shaft is currently being sunk in order to obtain a depth bulk sample of a small but high-grade pipe near Koidu. Grantham & Allen brieflly mention the discovery of a second kimberlite zone on the Tongo lease area, 30 miles south of Yengema : during the past 2 years a surface prospecting programme in this area has resulted in the identification of three dykes similar to those already exposed at Yengema. The economic significance of kimberlite dykes is two-fold: if of sufficiently high grade they may constitute economic orebodies in their own right and in addition they serve as guides to the presence of threedimensional pipes or 'dyke enlargements', which form more attractive mining targets than dykes owing to their greater tonnage for every foot of depth and the possibility of using cheaper larger-scale mining methods. Grantham & Allen's petrological study of kimberlites was based exclusively on diamond drill cores; as they point out, tropical weathering in Sierra Leone has