Minerais de manganês como contaminantes do minério de ferro na mina N5W em Carajás, Pará

Abstract

Mineralogical characterization and control of manganese and silica contents in the Carajás mine iron ore is a routine task. Relatively high concentrations of manganese and silica minerals, considered ore impurities, are present in the high grade iron ore (>65% Fe) of the N5W mine. This study aimed to identify these minerals and their mode of occurrence in the iron ore from N5W, improving the mine planning and exploitation. The main manganese minerals in the iron ore are: (a) pyrolusite, bixbyite, cryptomelane, hollandite, ramsdellite and chalcophanite, all Mn oxy-hydroxides; and (b) the braunite, a silicate. These minerals were found in hydrothermal veins and veinlets, cross-cutting the primary bedding of the iron ore and subordinately forming centimetre scale lens shaped veins, interbedded in the jaspilite layers. The Mn rich veins are spatially associated with fault zones and breccia bodies showing pyrolusite, chalcophanite, braunite and bixbyite veins, subordinated lenses of cryptomelane and hollandite. The presence of pyrolusite and cryptomelane indicates weathering of the rocks. Quartz and Fe-Mn oxy-hydroxides are the main minerals filling faults and fractures in the N5W mine, where MnO grades in veins, reach up to 61,74%. The Mn minerals in the iron ore are indicative of sedimentary and hydrothermal origin. Weathering caused alteration of the primary minerals, promoting mobilization and reprecipitation of new Mn minerals together with Fe oxy-hydroxides. Typical geochemical association in the Mn rich iron ore Mn-As-Cu-Zn-Ag with positive anomalous ETR levels, whilst in the iron formation Zr-Hf-Nb-Ta-Sc-Th with ETR negative values. The rare earth elements concentrations are higher in the Mn rich zones. These zones show strong positive Ce anomalies when hollandite is present. Higher SiO2 contents are related to the presence of metric jaspilite boulders surrounded by hematite friable, in the thick saprolite zone. Finally, the Mn contaminants are diverse in origin, however hydrothermal Mn rich minerals are dominant, closely associated with the proto-iron ore. The high SiO2 contents are indicative of the base of the alteration profile, incorporating part of the proto-ores. Keywords: Mn Oxi-hydroxides; hematite; quartz; silica; contaminants, Carajás Formation.