Specularitic Iron Ores and Shear Zones in the Quadrilatero Ferrifero District

摘要

In the eastern high-strain domain of the Quadrilatero Ferrifero, Brazil, hard and soft high-grade hematite ore may occur as schistose bodies in shear zones mainly related to the Cambotas-Fundao thrust system. These orebodies are almost entirely constituted of oriented specularite platelets that develop both as a product of crystal-plastic deformation of hematite and solution-precipitation processes. Both mechanisms interact in the partitioning of the strain, while solution-precipitation is responsible for the precipitation of hematite and localised leaching of silica resulting in the formation of syntectonic ores. The analyses of fluid inclusions from specularite crystals of the Conceicao and Timbopeba deposits, located in the northern and southernmost areas of the eastern high-strain domain of the Quadrilatero Ferrifero, were studied using infrared microthermometry and Synchrotron x-ray fluorescence. The results indicate the participation of highly saline aqueous H_2O-NaCl fluids containing other additional cations (Ca, Fe, K and probably Ti) in solution and homogenisation Temperature (Th) averaging between 130 deg - 190 deg C. However, the partial fusion of metastable saturation phases indicates crystallisation temperatures of approximately 400 deg C. The mineralisation process produces high-grade orebodies as well as Fe-rich specularitic itabirites that are explored in that area. The shear zones are related to thrust faults mapped and interpreted as related to the PanAfrican/Brasiliano Orogeny that affected the Quadrilatero Ferrifero, thrusting the Paleoproterozoic platform sequence of the Minas Supergroup over Mesoproterozoic quartzites of the Espinhaco Supergroup. They intersect and even overprint an early metamorphic and tectonic fabric of Paleoproterozoic age (Transamazonian Orogeny). The geological and fabric relationships lead to the conclusion that the fluids involved in the mineralisation were most probably of external origin, of still unknown source (possibly igneous), during the closure of the Sao Francisco Craton.