Genesis of lateritic iron ore from banded iron-formation in the Capanema mine (Minas Gerais, Brazil)

Abstract

Lateritic weathering of metamorphosed Proterozoic banded iron-formation (BIF), locally named itabirite in Brazil, produced large deposits of iron ore. At the Capanema iron-ore mine in the Quadrilatero Ferrifero in Minas Gerais state (southeast Brazil), the BIF is a bedded metasedimentary rock composed essentially of layers of hematite/kenomagnetite alternating with layers of quartz. The BIF is cut by dolerite dykes. The lateritic weathering of itabirite in wet tropical conditions resulted in a well-developed weathering mantle, with a specific succession of horizons of different chemical, mineralogical and textural compositions capped by a ferruginous duricrust or canga. The lack of alumina in the itabirite and the intense leaching of silica inhibited the formation of clay minerals. However, some alumina (from the weathered dolerite) is incorporated in secondary minerals. The process of alternating dissolution and reprecipitation of Fe and Al oxides and oxy-hydroxides controls the lateritic weathering. Eight different types of goethite (I–VIII) and three types of hematite (I–III) were identified based on texture, derivation and chemistry, related to five transformation fronts that include desilication, oxides dissolution, cementation, duricrust and erosion. Mass balance of the weathering column indicates varying iron redistribution (as goethite and hematite) through the profile, with an absolute enrichment of aluminium, locally derived from weathered dolerite. This alumina appears to act as an accelerant in the lateritisation process.