DOI: 10.1590/2317-4889201620160101 ARTICLE The Piriá aluminous lateritic profile: mineralogy, geochemistry and parent rock O perfil laterítico aluminoso do Piriá: mineralogia, geoquímica e rocha mãe Pabllo Henrique Costa dos Santos1*, Marcondes Lima da Costa2, Alessandro Sabá Leite3 ABSTRACT: Relatively small aluminous lateritic deposits are abundant RESUMO: Depósitos lateríticos de pequeno porte são frequentes no in the northeast and northwest parts of the Pará and Maranhão states, nordeste do Pará e noroeste do Maranhão. A maioria está mineralizada respectively. Most of them hosts aluminum phosphate mineralization for- em fosfatos de alumínio e ocorre em morros e platôs, que se destacam na ming hills and plateaus that stand out in the topography of the undulating planície rebaixada dessa região. A Serra do Piriá é uma dessas feições, plains of this region. The Piriá ridge is one of those topographic features, estando recoberta por couraças ferruginosas, alvo de explotação na década covered by lateritic iron crusts that have been studied in the 1970s as part de 1970 e de recente prospecção de fosfatos. Este trabalho buscou aprofun- of iron ore exploration campaigns and recently for phosphates prospec- dar o conhecimento sobre a evolução laterítica do depósito Piriá e demon- tion. This study improves the knowledge about the evolution of the late- strar sua compatibilidade com os lateritos mais evoluídos da Amazônia, ritic Piriá deposit and demonstrates its relationship with the most evolved conhecidos como maturos, os quais constituem grandes mineralizações laterites of the Amazon, known as mature laterites, which formed major formadas durante o paleógeno. Investigaram-se amostras de um furo de ore deposits during the Paleogene. Samples of a 17 meter-deep borehole sondagem com 17 m de profundidade, por meio de análises mineralógicas were investigated through mineralogical (X-ray diffraction —XRD, opti- (difração de raios x — DRX, microscopia ótica e eletrônica) e químicas cal and electron microscopy) and chemical methods (inductively coupled (espectrometria de massa por plasma acoplado indutivamente — ICP- plasma mass spectrometry — ICP-MS, inductively coupled plasma opti- MS, espectrometria de emissão óptica com plasma indutivamente acop- cal emission spectrometry — ICP-OES and X-ray fluorescence — XRF). lado — ICP-OES e espectrometria de fluorescência de raios X — FRX). The studied lateritic profile comprises a clay bauxitic horizon overlaid by an O perfil compreende horizonte argiloso bauxítico recoberto por crosta fer- aluminous iron crust. Upwardly continuous dissolution of kaolinite occurs roaluminosa. Ascendentemente ocorre contínua dissolução da caulinita with the formation of gibbsite, as the result of intensive leaching, resulting com formação de gibbsita, resultante de intensa lixiviação, de modo que in a higher Al2O3 content in the crust. The continuous formation of he- os teores de Al2O3 são superiores na crosta. Também se verificou contínua matite from goethite resulted from the transition to more arid conditions. formação de hematita a partir de goethita, resultante de transição para Anatase is a newly formed mineral (100–400 nm crystallites), showing a condições mais áridas. Anatásio neoformado (em cristalitos de 100 a 400 gradual increase, following the increase in TiO2 content, which is high nm) apresenta maior abundância em direção ao topo, acompanhando os and indicative of a mafic parent rock, confirmed by the Ti × Zr dispersion teores de TiO2, que são elevados e remetem à rocha mãe, de composição pattern. Prominent zoning in the lateritic profile is characterized by the máfica, confirmada pelo padrão de dispersão Ti × Zr. A mineralização mineralization in bauxite and augelite and abrupt chemical transition be- em bauxita e augelita e a transição composicional brusca entre os hori- tween the horizons, marked by a decrease in Si and increase in Fe content zontes, principalmente quanto aos teores decrescentes de Si e crescentes de from the bottom to the top of the profile. These features are compatible Fe em direção ao topo, demonstram o forte zoneamento típico dos lateritos and indicative of mature laterites formed in Amazon during the Paleogene. maturos, formados na Amazônia durante o paleógeno. KEYWORDS: Gurupi region; mature laterites; bauxite. PALAVRAS-CHAVE: região do Gurupi; lateritos maturos; bauxita. 1Geosciences Museum, Institute of Geosciences, Federal University of Pará – UFPA, Belém (PA), Brazil. E-mails: [email protected], [email protected] 2Program for Post-graduation in Geology and Geochemistry, Institute of Geosciences, UFPA, Belém (PA), Brazil. E-mails: [email protected], [email protected] 3Vale Institute of Technology – ITV, Belém (PA), Brazil. E-mails: [email protected], [email protected] *Corresponding author. Manuscript ID: 20160101. Received in: 08/17/2016. Approved in: 09/23/2016. 617 Brazilian Journal of Geology, 46(4): 617-636, December 2016 The Piriá Aluminous laterites INTRODUCTION higher relief in the form of hills or ridges that stand out in the regional undulating plain. Lateritic deposits are found in almost the entire Amazon The Gurupi region’s main lateritic deposits, which host region. Among the main identified Amazonian lateritic phosphate mineralization and are sometimes associated with zones, the Gurupi region (northeastern Pará and northwes- bauxite, include the Sapucaia, Boa Vista, Jandiá, Cansa Perna, tern Maranhão, extending until the coast) stands out for its Itacupim, Pedra Grande, Piriá and Peito de Moça (in the state compositional diversity and metallogenic potential, mainly of Pará); and the Pirocaua, Trauira and Tromaí (in the state of for phosphates, manganese, gold, titanium and aluminum Maranhão) (Fig. 1A). The most significant deposits in terms (Costa 1997). In previous years, geological exploration for of volume and P2O5 content are the Sapucaia (that recently phosphates has intensified because of the Brazilian demand began to be mined) (Leite 2014), Itacupim, Pirocaua and for fertilizers, which depends on large-scale imports. Primary Trauira (Oliveira & Costa 1984). and secondary gold searches continue to occur. Most of the lateritic deposits from the Gurupi region are Physiographic and geological context classified as mature. This term represents evolved laterites Piriá ridge is located along the left border of the homony- with more individualized and complex horizons, recogni- mous river, in the municipality of Vizeu, northeastern Pará. zed by typical textures, structures and mineral-geochemical The ridge comprises a chain of N–S-oriented plateaus, which aspects (Costa 1991). Generally, these deposits comprise are 6 km-long in length and 178 m high, near the border of A 1º00'00"S 1º00'00"S Legend Road City Lateritic deposit River 2º00'00"S 2º00'00"S 48º00'00"W 47º00'00"W 46º00'00"W 45º00'00"W 178 C B 150 Legend Quaternary sediments 150 Tertiary sediments Phillites Elevation (m) Medium grade metamorphic rocks 100 Volcanic rocks Quartz veins Granites, migmatites and gneiss 0 Tonalites, trondhemites and granites -23 Lateritic crust Plateau outline 2.5 km 7.5 km 12.5 km Foliation Village Road River 0 4 km º03'47" º03'47" 1 1 46º21'35" 46º11'13" Figure 1. (A) Phosphate and bauxite deposits located along the coast of northeastern Pará and northwestern Maranhão, with Piriá ridge highlighted (Costa & Sá 1980, Costa 1980; Costa 1990, Costa 1997); (B) three- dimensional Piriá ridge, generated with Global Mapper 14 software from digital elevation model (Shutte Radar Topography Mission); (C) Piriá ridge and its surrounding geology, modified from Costa (1982). 618 Brazilian Journal of Geology, 46(4): 617-636, December 2016 Pabllo Henrique Costa dos Santos et al. Piriá river (Fig. 1B). The overlaying aluminous iron crust was The optical microscopy analyses were conducted with partially mined during the 1980s and 1990s by Cimentos a Zeiss microscope, Axio Lab Pol model, to determine do Brasil S/A (CIBRASA) as iron ore, added to the limes- the textural and some mineral aspects from the lateritic tone that was extracted near Capanema (Pará) to produce profile. The adopted nomenclature for the texture was cement. The altitude in the area of exploitation was 160 m. that proposed by the Atlas of Micromorphology of Mineral The ridge relief is sustained by lateritic crusts. Phyllite, Alteration and Weathering (Delvigne 1999). The mineral schist and local volcanic rocks and quartz veins outcrop and textural aspects were captured with a Canon digital along the ridge slopes (Fig. 1C). This lithotype associa- camera, model A460. tion matches the Aurizona Group, which was defined by The Fe-Al substitution in FeOOH-AlOOH (goethite-dias- Pastana (1995) as a greenschist to amphibolite facies meta- pore) solid solution was determined according to Thiel’s (1963) volcanosedimentary sequence that comprises various schists, method, which evaluates the positioning variations of d111, phyllites, meta-pyroclastic rocks, meta-chert and some meta- d130, d140 and d021 goethite reflections according to the mafic and meta-ultramafic rocks. Klein and Moura (2001) AlOOH molar content. defined 2240 ± 5 Ma as the age for this unit. Anatase is a typical lateritic mineral (Costa et al. 1991, Granites, granodiorites and trondhjemites also occur Oliveira et al. 2013, Giorgis et al. 2014), also present in around Piriá ridge. These rocks match the Tromaí intrusive Piriá ridge deposit. So there was an attempt to make a suite (Klein et al. 2005), which has great geographic exten-