DISCOVERY OF A KIMBERLITE PIPE AND RECOGNITION OF A DIAGNOSTIC BOTANICAL INDICATOR IN NW LIBERIA

Abstract

Diamond-bearing kimberlite dikes (fissures) are present in the deeply eroded Man Shield of West Africa. Small kimberlite pipes, generally less than 1 hectare in area, are known in Guinea, Sierra Leone, and Liberia. Exploration for larger bodies has been severely hampered by thick tropical vegetation, and the lack of distinct geophysical contrasts between weathered kimberlite and the nonresponsive nature of country-rock granites and granodioritic gneisses. Following several years of intense exploration in the highly active artisanal diamond district of northwestern Liberia (which was a major source of alluvial “blood diamonds”) by several large companies and the present study, we report that an elusive diamond-bearing kimberlite pipe has finally been located. A bonus to the pipe location is that an unusual botanical indicator, Pandanus candelabrum , is now recognized exclusively on the pipe and not in eluvium covering the adjacent kimberlite dikes. Plants ( Lychnis alpina ) have been widely used since medieval times for copper in Sweden, and with Haumaniastrum katangese, more recently in Africa. Other plants have evolved to physiologically stabilize heavy metals (U, Pb, Zn, Ni, Cr, Ba, Pb, Zn) in leaves and bark. Termite hills have been used in diamond exploration for kimberlitic indicator minerals (ilmenite, chromite, garnet, pyroxene) in Botswana, the United States, and Australia, but the identification of Pandanus candelabrum, with stilt-like aerial roots, is the first plant to be described that has a marked affinity for kimberlite pipes. This could dramatically change the exploration dynamics for diamonds in West Africa, as geobotanical mapping and sampling is cost-effective in tough terrain.