Abstract We present this manuscript as a methodological approach and general guidelines for geochemical mapping and background/baseline projects for environmental assessment in tropical areas. A case study was carried out in the Itacaiúnas River watershed (IRW), Eastern Amazon, to fill in a gap in knowledge on the distribution of chemical elements, particularly those potentially toxic, in the near-surface environment of the area. The high-impact results of this research project revealed the need for similar scientific investigation across the globe with the implementation of a systematic methodology. The study shows, for example, the importance of well-planned field activities, multi-medium sampling, analytical methods, laboratory procedures, database construction, and general aspects of data processing and statistical treatment. The importance of this contribution is that it can be used as a reference in support of geospatial analysis in research within the scope of geochemical mapping and background-baseline projects. The database is accessible through a web-based geographic information system front-end; a Geochemical Atlas of the IRW will be available as soon as possible.

RESUMO: Depósitos lateríticos de pequeno porte são frequentes no nordeste do Pará e noroeste do Maranhão. A maioria está mineralizada em fosfatos de alumínio e ocorre em morros e platôs, que se destacam na planície rebaixada dessa região. A Serra do Piriá é uma dessas feições, estando recoberta por couraças ferruginosas, alvo de explotação na década de 1970 e de recente prospecção de fosfatos. Este trabalho buscou aprofundar o conhecimento sobre a evolução laterítica do depósito Piriá e demonstrar sua compatibilidade com os lateritos mais evoluídos da Amazônia, conhecidos como maturos, os quais constituem grandes mineralizações formadas durante o paleógeno. Investigaram-se amostras de um furo de sondagem com 17 m de profundidade, por meio de análises mineralógicas (difração de raios x - DRX, microscopia ótica e eletrônica) e químicas (espectrometria de massa por plasma acoplado indutivamente - ICP-MS, espectrometria de emissão óptica com plasma indutivamente acoplado - ICP-OES e espectrometria de fluorescência de raios X - FRX). O perfil compreende horizonte argiloso bauxítico recoberto por crosta ferroaluminosa. Ascendentemente ocorre contínua dissolução da caulinita com formação de gibbsita, resultante de intensa lixiviação, de modo que os teores de Al2O3 são superiores na crosta. Também se verificou contínua formação de hematita a partir de goethita, resultante de transição para condições mais áridas. Anatásio neoformado (em cristalitos de 100 a 400 nm) apresenta maior abundância em direção ao topo, acompanhando os teores de TiO2, que são elevados e remetem à rocha mãe, de composição máfica, confirmada pelo padrão de dispersão Ti × Zr. A mineralização em bauxita e augelita e a transição composicional brusca entre os horizontes, principalmente quanto aos teores decrescentes de Si e crescentes de Fe em direção ao topo, demonstram o forte zoneamento típico dos lateritos maturos, formados na Amazônia durante o paleógeno.

ABSTRACT: Relatively small aluminous lateritic deposits are abundant in the northeast and northwest parts of the Pará and Maranhão states, respectively. Most of them hosts aluminum phosphate mineralization forming hills and plateaus that stand out in the topography of the undulating plains of this region. The Piriá ridge is one of those topographic features, covered by lateritic iron crusts that have been studied in the 1970s as part of iron ore exploration campaigns and recently for phosphates prospection. This study improves the knowledge about the evolution of the lateritic Piriá deposit and demonstrates its relationship with the most evolved laterites of the Amazon, known as mature laterites, which formed major ore deposits during the Paleogene. Samples of a 17 meter-deep borehole were investigated through mineralogical (X-ray diffraction -XRD, optical and electron microscopy) and chemical methods (inductively coupled plasma mass spectrometry - ICP-MS, inductively coupled plasma optical emission spectrometry - ICP-OES and X-ray fluorescence - XRF). The studied lateritic profile comprises a clay bauxitic horizon overlaid by an aluminous iron crust. Upwardly continuous dissolution of kaolinite occurs with the formation of gibbsite, as the result of intensive leaching, resulting in a higher Al2O3 content in the crust. The continuous formation of hematite from goethite resulted from the transition to more arid conditions. Anatase is a newly formed mineral (100-400 nm crystallites), showing a gradual increase, following the increase in TiO2 content, which is high and indicative of a mafic parent rock, confirmed by the Ti × Zr dispersion pattern. Prominent zoning in the lateritic profile is characterized by the mineralization in bauxite and augelite and abrupt chemical transition between the horizons, marked by a decrease in Si and increase in Fe content from the bottom to the top of the profile. These features are compatible and indicative of mature laterites formed in Amazon during the Paleogene.