Geochemistry of rare-earth elements in surface lateritic rocks and soils from the Maicuru complex, Para, Brazil

Abstract

The Maicuru area contains a typical alkaline/ultramafic rocks-carbonatite complex that is part of the Guyana Craton, northern Brazil, and exhibits thick lateritic profiles. Tops of profiles are hardened ferruginous horizons frequently called “iron crust” or “ferricrete” which gives rise to landforms called “iron-hat”. The area is being explored for titanium and phosphate resources by Docego, a subsidiary of Companhia Vale de Rio Doce (CVRD). This paper discusses rare-earth elements (REE) mineralization related to aluminum phosphates in Ti-rich surface laterites (iron crusts and lateritic soils) containing anatase over the Maicuru Complex. Some samples show total REE concentrations as high as 20.0 wt.% that is contained in crandallite-goyazite-florencite solid solutions, with the last end member predominant. Mean Σ REE contents are slightly greater in soils than in iron crusts. Chondrite-normalized plots for REE in soils and iron crusts reveal strong fractionations with high (La/Lu) N mean values, where the heavy rare-earth elements (HREE) are strongly depleted compared to light rare-earth elements (LREE), a pattern typical of carbonatite and alkaline rocks. Strong positive and negative Ce anomalies were observed in iron crust samples, but were less common in soils. Lateritic iron crusts distinctly differ from lateritic soils in geochemical properties. Lateritic iron crusts over mineralized rocks are enriched in Ti, Fe, P, REE and resistate minerals, but soils are not. Anomalous concentrations of REE in laterites strongly indicates the presence of phosphates in laterites and in parent rocks. The study illustrates the importance of understanding laterization processes for geochemical exploration in tropical terrains.