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Geology and geochemistry of the Águas Claras and Pico Iron Mines, Quadrilátero Ferrífero, Minas Gerais, Brazil

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Abstract

The Águas Claras and Pico Mines are two world-class iron-ore mines hosted within the Lower- Proterozoic banded iron-formations (locally known as itabirites) of the Minas Supergroup located in the Quadrilátero Ferrífero district, Minas Gerais, Brazil. The Águas Claras orebody consists of a 2,500-m-long roughly tabular-shaped lens hosted within the dolomitic itabirite of the Cauê Formation. Dolomitic itabirite is the protore of the soft high-grade iron ore, which is the main ore type of the Águas Claras orebody, representing about 85% of the 284 Mt mined since 1973, with the remaining 15% comprising hard high-grade ore. Hematite is the main constituent of the iron ores. It occurs as martite, granular hematite and locally as specularite. Magnetite appears subordinately as relicts within martite and hematite crystals. Gangue minerals are very rare. These consist of dolomite, chlorite, talc, and apatite, and are especially common in contact with the protore. This virtual absence of gangue minerals is reflected in the chemistry of ores that are characterized by very high Fe contents (an average of 68.2% Fe).

The Pico orebody is a continuous ~3,000-m-long body of a lenticular shape hosted within siliceous itabirite, which is the protore of the soft high- and low-grade ores at the Pico Mine. The soft high-grade ores, together with the low-grade ores, called iron-rich itabirite, are the main types of ore, and respectively represent approximately 51 and 29% of the reserves. The remaining 20% consists of hard high-grade ore. The iron oxide mineralogy is the same as that of the Águas Claras Mine, but in different proportions. Gangue minerals are very rare in the high-grade ores, but are slightly more common in the iron-rich itabirite. Quartz is the dominant gangue mineral, and is found with minor quantities of chlorite. The chemistry of the high-grade ores is characterized by high Fe contents (an average of 67.0%) and low P, Al2O3, and SiO2, which are concentrated in the fines. Iron-rich itabirites average 58.6% Fe and 13.5% SiO2.

The genesis of the soft high-grade ores and iron-rich itabirites is related to supergene processes. Leaching of the gangue minerals by groundwater promoted the residual iron enrichment of the itabirites. This process was favored by the tropical climate and topographic situation. The original composition of the itabirites and the presence of structures controlling the circulation of the groundwater have influenced the degree of iron enrichment. The hard high-grade ores are of a hypogene origin. Their genesis is attributed to hydrothermal solutions that leached the gangue minerals and filled the spaces with hematite. This process remains a source of debate and is not yet fully understood.

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Acknowledgements

This paper is an integral part of the senior author´s Ph.D. thesis at the Instituto de Geociências of the Universidade de São Paulo–USP and is published by permission of Minerações Brasileiras Reunidas (MBR). C.A.S. thanks MBR for providing financial support and access to its database and laboratories. He is especially thankful to Mr. Oscar Tessari for his support and permission for publication of this work. We are sincerely grateful to Dr. Harry Fisscher for a first review of this paper in manuscript. We thank also the reviewers H.J. Dalstra and M.E. Barley for their careful review of the manuscript and most valuable suggestions.

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  1. Mineraçōes Brasileiras Reunidas-MBR, Av. de Ligação 3580, 34000-000 , Nova Lima, Brazil

    Carlos Alberto Spier

  2. Instituto de Geociências, Cidade Universitária, Universidade de São Paulo, Rua do Lago 562, 05508-900 , São Paulo, Brazil

    Sonia Maria Barros de Oliveira

  3. Instituto de Geociências, Campus da Pampulha, Universidade Federal de Minas Gerais, 31270-901 , Belo Horizonte, Brazil

    Carlos Alberto Rosière

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  1. Carlos Alberto Spier
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Spier, C.A., de Oliveira, S.M.B. & Rosière, C.A. Geology and geochemistry of the Águas Claras and Pico Iron Mines, Quadrilátero Ferrífero, Minas Gerais, Brazil. Miner Deposita 38, 751–774 (2003). https://doi.org/10.1007/s00126-003-0371-2

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