Ferricrete genesis and supergene gold behaviour in Burkina Faso,West Africa

Abstract

The lateritic weathering process of Precambrian albitite formations containing gold sulphide mineralization has been studied at Larafella, southern Burkina Faso. The regional landscape consists of three land surface units with respective altitudes of 300 m, 280 m and 265–250 m. Plateaus on a barren schist formation form the first two upper units; there are remnants of two old planation surfaces capped by ferricretes. The third unit is a large recent pediment covered by a soft ferricrete. Within the weathering profile of this lower surface, preservation of the parent rock structures, Au and As content, Au particle morphology and the Ag content indicate the inheritance of the parent mineralized rock. The preservation of unweathered sulphides within the soft ferricrete and their absence in the underlying saprolite indicate that the ferricrete formed on a stripped surface at the expense of a slightly weathered rock, and preceded the formation of the clay horizons and the deepening of the weathering profile. Some sulphide grains and Au particles affected by transportation marks are also present within the ferricrete of the upper units. Transported bauxitic and ferruginous debris of previous lateritic formations have also been described. The upper units are accumulation surfaces, each with 8–10 m of reworked material on the bedrock. The results provided here improve our knowledge of the geomorphic dynamics that can be useful for Au exploration in lateritic environments.