New Ore Types from the Caue Banded Iron Formation, Quadrilatero Ferrifero, Minas Gerais, Brazil - Responses to the Growing Demand

摘要

The mining district of the Quadrilatero Ferrifero (QF) in southeastern Brazil contributes to 70 per cent of the Brazilian iron ore production, which reached 370 Mt in 2010. Traditionally, four iron ore types were mined in the QF region: 1. soft haematite, 2. hard haematite, 3. friable 'itabirite' (the Brazilian name for metamorphosed banded iron formation), and 4. canga (an indurated iron-rich crust). With the exception of the hard haematite, these categories derive from supergene enrichment of the Paleoproterozoic Caue Itabirite. High current iron ore demand means that the QF high grade supergene ores will be depleted in a few decades. Consequently, several mining projects are being developed to exploit unenriched, fresh itabirite. Among them, the Mineragao Usiminas Serra Azul project in northwestern QF is one of the most advanced. Four major itabirite types occur in the QF region: 1. siliceous (or standard), 2. dolomitic, 3. amphibolitic, and 4. magnetitic. The dominant type in Serra Azul region is a fresh siliceous itabirite, composed essentially of haematite, martite and quartz that averages 36 per cent Fe (51.5 per cent Fe_2O_3) and 47 per cent SiO_2. The second type, the magnetitic itabirite, consisting of magnetite, martite, grunerite-cummingtonite, quartz, dolomite, stilpnomelane, and subordinate haematite, is typically a strongly magnetic greenish banded iron formation with the iron grade varying between 25 and 35 per cent. Chemically, it differs from the siliceous itabirite due to the relatively high CaO, MgO and, consequently, high LOI contents. Together with supergene ores, fresh itabirites have been included in the ore reserves of the Serra Azul project. We anticipate that all the four types of fresh itabirite will be incorporated as ore in the QF region, causing a huge increase on the iron ore resources of the district, and representing enormous environmental and technological challenges for the local iron ore industry.