Geochemical and mineralogical characterization of a section through the nickeliferous laterite in Fazenda da Roseta, Liberdade, Minas Gerais, Brazil

Abstract

Geochemical and mineralogical studies undertaken through a section of the nickeliferous laterite profile in Fazenda da Roseta, Liberdade, Southern Minas Gerais, revealed mineable grades, both in the saprolitic horizon (A-type ore) and in the oxidized one (C-type ore). The A-type ore, well defined in the innermost 1.5 m of the section, boasts representative grades at the order of 1.7 wt.% NiO, with amorphous garnierite-type minerals, besides chlorite and likely serpentine, as the Ni-rich phases. An approximately 2 m-long interval defines a transitional zone from the silicatic toward the oxidized ore, marked by the outward gradual absorption of residual blocks from the parent rock within a goethitic plasma. Impregnations and stockwork-like veins of asbolane, confirmed by local Mn and Co anomalies, delineate the outer edge of this zone. The oxidized ore stretches, based on Ni contents, for roughly 4 m. Average grades lie about 1.2 wt.% NiO, containing goethite as the main Ni ore mineral, plus some secondary talc. Ever since the innermost part of the section, Ni has been continuously washed away, in such a manner that its concentrations become rather low in the outermost part. Anomalous geochemical signatures may indicate the occurrence of dykes of uncommon igneous rocks crosscutting the profile. The absence of smectites in the profile points out that free flow conditions have prevailed, which is reinforced by the substitution of kaolinite by gibbsite, which highlights severe hydrolysis conditions (tendency to alitization). Kaolinite veins in the gibbsite-only zone suggest post-lateritic origin. The laterite development determines an evolution in pH conditions from slightly alkaline in the early stages (quartz–kaolinite–zeolite association) to mildly acidic in the more advanced phases (presence of Al-goethites).