Aspectos mineralógicos geoquímicos da laterita niquelífera da área do Vermelho, Serra dos Carajás

Abstract

The mineralogical-geochemical study of nickel-bearing laterite from the Vermelho area, Serra dos Carajás, was based on 64 samples obtained from a drill in the mother rock and a well in the alteration profile. The lateritic profile, developed on serpentinized ultrabasic rocks, consists of three distinct horizons; from the base to the top: saprolithic horizon, with more than 4 meters of thickness, clayey horizon, with 6.1 meters, and limonitic horizon with 8.3 meters. Mineralogically, the saprolitic horizon consists of serpentine, chlorite, quartz, hematite and goethite, the clayey horizon of smectite, chlorite and goethite and the limonitic horizon of quartz, goethite, hematite, chlorite and kaolinite. Maghemite is present from the base to the top of this profile. During the action of weathering, the main and trace elements underwent leaching, transport and reprecipitation processes, resulting in the concentration of these elements in the different horizons of the profile, with the exception of magnesium, which was leached to a large extent. Despite the low mobility of chromium in the lateritic medium, this element was leached from the saprolitic horizon and from there it enriches itself, thus behaving as a relatively mobile element. The profit and loss calculations were difficult to make because no element showed continuous relative enrichment from the bottom to the top of the profile. Considering the average contents of the horizons of the lateritic profile and fresh serpentinite, only iron and cobalt present continuous relative enrichment. The nickel concentration in the clayey horizon possibly occurred through an ion exchange process between the smectite and the level migrated by descending solutions, after its release from goethite. The parent rock contains 0.3% Ni, while the clayey horizon has an average content of 4.5% Ni. Cobalt and zinc were also strongly enriched during the laterization process. The cobalt was concentrated in the limonitic horizon, where it reaches an average content of about 0.1% by weight, thus suffering an enrichment of almost nine times in relation to its content in the mother rock. The distribution of cobalt in the lateritic profile is controlled by iron and manganese minerals. Zinc was concentrated in the saprolitic horizon, where it has an average content of approximately 0.05% by weight, enriching about 10 times its content in the mother rock.