Geology and geochemistry of the Aguas Claras and Pico Iron Mines, Quadrilatero Ferrifero, Minas Gerais, Brazil

摘要

The Aguas 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 Quadrilatero Ferrifero district, Minas Gerais, Brazil. The Aguas Claras orebody consists of a 2,500-m-long roughly tabular-shaped lens hosted within the dolomitic itabirite of the Caue Formation. Dolomitic itabirite is the protore of the soft high-grade iron ore, which is the main ore type of the Aguas Claras orebody, representing about 85% of the 248 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 the 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 Aguas Claras Mine, but in different proportions. Gangue minerals are very rare in the high-grade ores, but the 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, Al_2O_3, and SiO_2, which are concentrated in the fines. Iron-rich itabirites average 58.6% Fe and 13.5% SiO_2. 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.