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Petrografia, geoquímica e assinatura isotópica de PB de formações ferríferas associadas à gênese das cavernas da Serra Sul, Carajás – PA
(Universidade Federal do Pará, 2016-06-17) CABRAL, Erica da Solidade; MACAMBIA, Joel Buenano; http://lattes.cnpq.br/4842128592488825; TORO, Marco Antonio Galarza; http://lattes.cnpq.br/8979250766799749
Geospeleological studies in hosted caves in banded iron formation (banded iron formation - BIF's), type jaspilite, which is the iron protore the S11D body, Serra Sul, showed that these rocks are at different stages of weathering, allowing classifies them into three groups: 1) non-altered jaspilite, 2) altered jaspilite and 3) Iron Ore. Samples BIF’s were collected both in caves and in correlated drillholes stratigraphically to S11D-0035 cave. This stratigraphic correlation was established from the construction of geological section, which allowed us to determine the likely initial lithological setting of the cave and its stratigraphic level. It was concluded that the cave is in the lower portion of the Carajás formation near the contact with the Parauapebas Formation. Petrographic observations showed that jaspilite is characterized by alternating centimetric bands of opaque mineral composed of hematite-1, magnetite and maghemite and subordinate siliceous bands formed by chert and granular quartz. While the non-altered jaspilite consists predominantly of hematite-2 and subordinately hematite-1 and magnetite, banding is absent because the band of silica minerals, which is more soluble, it was almost in its entirety leached, which led to the generating dissolution cavities. In addition, they also observed neo-formed mineral phases, goethite and hematite-3. The chemical analysis of rocks of different groups corroborate the petrographic evidence. Samples of non-altered jaspilite SiO2 content ranges from 40.0 to 44.5% and Fe2O3T content from 53.9 to 58.3%, and the other oxides present, Al2O3, MnO, P2O5, TiO2, CaO, MgO, Na2O, K2O, have very low levels, as well as in other types. In contrast, the rocks are classified as altered jaspilite higher concentration of iron (from 87.9 to 97.1%) and lower SiO2 content (0.3 to 1.1%). The iron ore classified as representing the most advanced stage change, has a high content of Fe2O3T, ranging from 96.2 to 98.3% and low amounts of SiO2 from 0.4 to 1,0%. In all three groups of rock a low content of ETR was observed and, from bottom to top of the weathering profile is observed an increase in the amount of these elements, from 6 to 18 ppm. The ratio La/Eu ranges from 1.3 to 2.2 ppm non-altered jaspilite, from 0.2 to 1.6 ppm altered Jaspilite and 0,1 to 0,8 ppm Iron Ore. This decrease in iron ore may be due to less mobile behavior of HREE relative to LREE in the advanced stages of weathering. The positive anomaly Eu (Eu/Eu* > 1), typical of the BIF's was observed in all samples from different groups change. The Pb-Pb isotopic analyzes in non-altered jaspilite indicate 206Pb/204Pb ratios ranging from 15.247 to 26.111, the ratio 207Pb/204Pb ranges from 15.292 to 16.300, and the ratio 208Pb/204Pb ranges from 34.596 to 37.614. The figures suggest that these rocks are less radiogenic and have isotopic signature similar to the upper crustal rocks. In jaspilite Changed the isotopic ratios 206Pb/204Pb ranging from 16.827 to 23.244, the 207Pb/204Pb from 15.635 to 16.279, and 208Pb/204Pb from 34.715 to 38.811. In Iron Ore the 206Pb/204Pb ratios ranging from 15.702 to 22.845, 207Pb/204Pb from 15,369 to 16,221, and 208Pb/204Pb from 35,169 to 38,467. Therefore, changes were observed in the isotopic signature over change profile suggesting that there were no metamorphic events, tectonic reactivations, percolation of hydrothermal fluids or any other event that upset the Pb-Pb geochronometer and would add materials (Pb) come from other sources. The genesis and evolution of the caves, in addition to chemical processes, relief plays an important role, acting at two different times. First, facilitating the infiltration and percolation of rainwater and channel them to underground portions of the plateau, causing the dissolution of the siliceous layer and the formation of iron ore, loss of rock volume, reduction of layers and concomitant formation of caves. In a second step these caves are exposed by relief denudation processes that favor the retreat of the slopes. Therefore, the analysis of chemical processes with the help of petrography and geochemistry showed that the dissolution and leaching processes are the primary processes in the formation of caves linked to erosive processes provide its evolution. The analysis of chemical processes indicates common origin (weathering) for both the caves and for iron ore, probably contemporary.