High-grade iron ores in the laterite weathering crust after the banded iron formation in the Simandou mountain range, Republic of Guinea

Abstract

In West Africa, in the southeastern Guinea, in the Simandou Range region underlain by the Archean gneiss-granite basement, a north–south synclinorium structure is extended, that is composed of the Lower Proterozoic series of metamorphic rocks. A thick sequence of the banded iron formation (itabirites) with interlayers of phyllites and mica schists is exposed in cores of the synclines. According to their composition, the itabirites are subdivided into quartz-magnetite and amphibole-quartz-magnetite varieties. High-grade iron ores (60–66% Fe) formed in a pseudomorphic manner after the itabirites in the Cenozoic lateritic weathering crust. At the North Simandou, Zogota, and Pic-de-Fon deposits, the rich ores compose mantle-shaped orebodies up to 150–350 m deep. The orebodies possess the following vertical zonality: primary magnetite itabirites – martitized itabirites – quartz-martite friable ore – martite friable ore and martite–limonite friable ore – goethite-hematite friable to solid ore – deluvial limonite ore (kanga and cuirass). All the types of the high-grade ores are dominated by hematite. The rich iron ores were formed from itabirites as a result of the oxidation of magnetite to martite, the almost complete removal of silica, and the subsequent additional supply of iron and its precipitation in the form of hydroxides. During the formation of the iron ores, kaolin clays and bauxites were formed after the phyllite interlayers.