Mineralogia e geoquímica de perfis lateríticos no nordeste do Estado do Amazonas

Abstract

Presidente Figueiredo township is located on the northeastern Amazonas State. Geologically, it plays a major role, due to exposing profiles of different units with access favoured by highway BR-174, which links Manaus (AM) to Boa Vista (RR). It also plays a fundamental role on the State s economy, since it harbours a valuable mining province called Pitinga mine, situated nearly 300 km from Manaus, one of the main deposits of tin, rare minerals (Zr, Nb, Th, Ta, Y and ETR) and, criolite of the Amazonian Craton. The purpose of this study is to investigate the effects of laterisation on the sedimentary rocks of Prosperança Formation in the Presidente Figueiredo region and in the volcanic rocks of Iricoumé Group and granite rocks of Mapuera Intrusive Suite in the Pitinga Mine area. Thus, besides contributing to the region s paleoclimatic reconstruction, it allowed to evaluate Pitinga bauxite economical potentiality and verify the potential of utilizing the paleomagnetism technique on lateritic crusts as a relative dating tool of the supergenic processes occurring in Amazonia. Three profiles (P1, P2 and P3) were selected and their colour, textural and structural parameters studied. Mineralogical identification associated to the quantification of the major elements allowed for the semi-quantitative calculation of the main residual/supergenic minerals and bauxite potentiality evaluation. Different crust face petrographic laminas were made in order to aid on the mineralogical, structural and textural description. Trace elements and rare earths were also analyzed in order to investigate possible mineralizing and fractionating processes. The paleomagnetism technique was used to evaluate its potential use on lateritic crusts as well as to estimate the lateralization relative age in the region. Profile 1 is constituted from the base to the top by the paprolite, mottled horizon, massive to protonodular crust (faces Fe-Al), dismantled and soil, profile 2 by saprolitic horizon, protonodular to pisolitic crust (faces: Al, Al-Fe, Fe-Al and Al), dismantled and soil, while profile 3, with structure similar to Profile 2, by the saprolite horizon, vermiform and nodular crust (facies: Al, Fe-Al, Al-Fe and Al), dismantled and soil. If on one hand the crust s Al face on profile 1 was not sighted, due it having been locally erode on P1A and not exposed on P1B, on the other hand, on profiles 2 and 3 it is well developed (~4 m thick). Regarding the mineralogy, gibbsite, kaolinite, hematite, goethite, quartz and anatase are the most abundant minerals on the three profiles, yet, the presence of muscovite and illite differentiates P1 from the remaining ones. Gibbsite contents are significantly higher on profiles 2 and 3, whereas hematite and goethite dominate on P1 crust, in spite of high contents also occurring on profiles 2 and 3 Fe-Al faces. In all of them quartz is very low, but for P1 sandy facies. Anastase is also low and doesn t differ from one profile to the other. Major element chemical analysis showed Al2O3, SiO2, Fe2O3, P.F., TiO2 and, subordinately, P2O5 to be the most abundant, being that on P1 SiO2 > Al2O3 > P.F. > Fe2O3, while on P2 Al2O3 > P.F > Fe2O3 > SiO2 and on P3 Al2O3 > P.F > SiO2 > Fe2O3. It all indicates that these differences were strongly influenced by the mineralogical and chemical caracteristics of the protolite, once the process acting on the three profiles was the same. Concerning the trace element analysis, findings showed that Cd, Bi, Ag, Tl, Se, Cs and Be occur on contents below the detecting limits, that Zr, Ba, V, Sr, Y, Hf, Mn, Pb, Sc, Zn, U, Sb, Cu and Co are more abundant on P2, Nb, Th, Ga, Sn, W, Ta, As, Mo, Au and Hg on P3, while Rb and Ni predominate on P1. Higher contents on profiles 2 and 3 result from the protolite characteristics, since they were formed from igneous rocks, generally carrying minerals containing these elements. ETR (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Eb and Lu) analysis showed the highest contents to be on P2, the lowest on P3, while on P1 they are intermediate. Upward concave sub-x parallel curves, with negative anomaly in Eu sharper on P3, were obtained following the contents normalization indicating higher fractionation of that element relative to P2, while for P1 the lacg of data from the parent rock prevent it from being compared to the others under this aspect. Distribution pattern on those elements also showed that lateritization promoted greater ETRL enrichment relative to ETRP on P1 than on P2, while on profile 3 there was ETRP enrichment relative to ETRL. P2 and P3 chemical findings indicated industrial quality for refractary bauxite, especially that from P2. Paleomagnetic analyses showed that the natural remaining magnetization (NRM) intensity and magnetic susceptibility in the samples from the crust developed over the sedimentary rocks of Prosperança Formation (P1) are greater than over the igneous rock of the Uatumã Supergroup (profiles 2 and 3). That behaviour is due to the higher content in ferromagnetic minerals such as hematite and Fe-Ti oxides in the crust of that profile. As to declination and inclination, despite the findings still being rather preliminary, due to the dispersion error being high, the mean directions (P1: 347º N/-32º, P2: 332º N/-48º and P3: 306º N/-35º) point N-NW, present similar inclination to the current pole and show good correlation with the poles from 50 and 70 Ma, in other words, of the Paleogene. The three studied profiles are graded as mature, since in spite of the Al face not having been sighted on P1 the textural, structural, mineralogical and geochemical characterisation suggests its existence, and therefore, along with P2 and P3, are complete profiles. It is possible they have developed in Upper Cretaceous/Eocene interval. This age is coherent with the climatic conditions to the formation of bauxites, once in Brazilian Amazonia as well as in the Guianas the wet climate was dominant, besides being an age suggested for the correlate deposits of the Paragominas (PA), Juruti (PA), French Guiana and Suriname region.