Sierra Leone forms the central part of the sundered West African craton whose counterpart forms the Guyana Shield of northern South America. Three Archean to early Proterozoic thermotectonic events and the early Phanerozoic Pan-African orogeny preceded the breakup of the original craton in the early Jurassic by rift faulting and the intrusion of tholeiitic magmas to form new oceanic crust which accompanied the opening of the Atlantic ocean.The Archean rocks of Sierra Leone consist of a granitic basement containing elements of early sedimentary and mafic formations and a group of supracrustal greenstone belts with banded ironstone and detrital sediments. These are mineralized with gold, chromite, tin, copper, nickel, antimony, arsenic, and iron ores and are intruded by molybdenum-bearing granites. Lateritic weathering and alluviation has led to economic concentrations of diamonds, bauxite, and lateritic iron ores and to heavy mineral concentrates, particularly rutile. Prospects for lateritic nickel deposits appear to be good.Little exploration has been carried out in the poorly exposed Rokel River Group of late Proterozoic age. This formation and also the Ordovician (?) Saoinya Scarp Group contain glacial sediments in their lower horizons. The Triassic-Lower Jurassic Free-town Layered Basic Complex lies upon the continental margin and was formed by successive pulses of basic magma which formed a rhythmic sequence of troctolite, gabbro, and anorthosite which in some cases ended with an iron-enriched ilmenite-magnetite horizon and acid veining. Some alluvial platinum has been won from this complex, but the source rock has not yet been identified.Kimberlite dikes were emplaced in the Cretaceous period and have been the sources of the country's diamond mining industry. The Tertiary and Pleistocene sediments are known to contain horizons carrying lignite and ceramic clays and may also contain exploitable concentrations of other minerals.