General: The Iron Quadrangle (Quadrilátero Ferrífero) is roughly between latitude 19-20°S and longitude 43-44°W. The district is a major iron, gold and gem producer. It was the principal source of bedrock gold mining until 1983 and accounts for about 40% of Brazil's total gold production from the 15th century until 2000. Two of the eight major new gold mines put into production between 1983 and 1990 are located in the Iron Quadrangle. The area between Itabira and Nova Era is an important gem mining district, and Itabira is also the most important iron mining district in the area.
Iron Deposits: The iron ore deposits are hosted within the Cauê Formation, a typical Banded Iron Formation (BIF) of the Lake Superior Type, which constitutes the intermediate chemical member of the Minas Supergroup, a Paleoproterozoic meta-sedimentary sequence. The BIFs, locally known as itabirites, are metamorphosed and strongly oxided. On the western border of the Iron Quadrangle, the Minas Supergroup is less deformed and preserves some original sedimentary structures. The metamorphism at this region reached the green schist facies. The iron ore bodies occur as discontinuous lenses of varied sizes and shapes within the itabirites. Two main types of iron ores are recognized: high-grade ore (Fe > 64%), called hematite ore and intermediate-grade ore (64% < Fe < 52%), called itabiritic ore. The intermediate-grade itabiritic ore is typically friable and generally grades to hard itabirite with low iron content. At the Águas Claras and Capão Xavier Deposits, the host itabirite was dolomitic and the ore associated is very friable, of very high-grade and low lump yield. At the Tamaduá, Capitão do Mato and Pico Deposits the host itabirite is siliceous.
The genesis of these ores is considered as due to a residual concentration of the iron oxides after leaching of the gangue minerals during the Cenozoic weathering. The genesis of the hard high-grade ore is more complex and not well understood. This ore type generally preserves the original banding of the host itabirite and shows hematite re-crystallization. Its origin is considered as hypogenic.
Gold Deposits: Gold is produced from numerous mines, mostly in the northern and southeastern parts of the district. The deposits are hosted in Archean or Paleoproterozoic carbonate- or oxide-facies BIFs or iron-rich cherts in supracrustal sequences. Consequently, some of the iron ore deposits are also worked for gold.
Gem Mines: The area between Itabira and Nova Era is the largest emerald mining district in Brazil, producing ca. 200 kg per month of facetable rough. In most cases, the beryllium needed for formation of the emerald deposits was provided by nearby pegmatites, which are consequently rich in beryllium and produce excellent gem-quality aquamarine and morganite.
References
- Morteani, G., Preinfalk, C., and Horn, A.H. (2000): Classification and mineralization potential of the pegmatites of the Eastern Brazilian Pegmatite Province. Mineralium Deposita 35, 638-655.
- Thorman, C.H., DeWitt, E., Maron, M.A.C., and Ladeira, E.A. (2001): Major Brazilian gold deposits - 1982 to 1999. Mineralium Deposita 36, 218-227. DOI 10.1007/s001260100170
- Lobato, L.M., Ribeiro-Rodrigues, L.C., and Reis Vieira, F.W. (2001): Brazil's premier gold province. Part II: Geology and genesis of gold deposits in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrifero. Mineralium Deposita 36, 249-277. DOI 10.1007/s001260100180
- Carlos A. Rosière & Farid Chemale Jr ITABIRITOS E MINÉRIOS DE FERRO DE ALTO TEOR DO QUADRILÁTERO FERRÍFERO – UMA VISÃO GERAL E DISCUSSÃO. GEONOMOS 8 (2): 27-43
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