Navegando por Assunto "Sedimentação e depósitos - Carajás, Serra dos (PA)"

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Estudos isotópicos (Pb-Pb, Sm-Nd, C e O) do depósito Cu-Au do Sossego, Província Mineral de Carajás
(Universidade Federal do Pará, 2006-10-30) NEVES, Marcely Pereira; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Sossego deposit is located at about 25 km NW of Canaã de Carajás, southeast of Pará, at the contact zone between the Grão Pará Group and basement rocks. This deposit comprises two main ore bodies, Sequeirinho and Sossego, which were formed in an environment where deformational processes, emplacement and cooling of intrusive bodies led to an intense hydrothermal fluid circulation. This work intended to characterize the ore host rocks, the mineralization styles and the types of hydrothermal alteration, as well as to date the sulfide mineralization and assess potential sources for metals and hydrothermal fluids. Invariably altered and deformed granitoids and mafic rocks, in addition to biotiterich rocks, hydrothermalites (magnetitites and ore bodies) and breccias occur in the deposit area. All these rocks are crosscut by mafic and felsic dikes. Albitization, epidotization, chloritization, silicification, amphibolitization and scapolitization were the main alteration processes recognized in the deposit. The first is more common in the granitoids while the last two are more typical of the mafic rocks. In these mafic rocks the concentration of actinolite increases towards the ore bodies resulting in the formation of actinolitites. A strong iron metasomatism was also identified, being responsible for the magnetitite generation. Abundant calcite-rich veinlets mark the final stage of the hydrothermal activity in the area. Although present in the Sequeirinho ore body, it was more intense in the Sossego ore body. Mineralization was subsequent to the shearing event and took place contemporaneously with the epidotization, chloritization and actinolization processes, allowing one to infer that mineralizating fluids, besides Cu, Au and P, were also enriched in Ca and Fe. Pb isotopic data in chalcopyrite from ore samples yielded ages of 2530 ± 25 Ma, 2608 ± 25 Ma (Sequeirinho ore body) and 1592 ± 45 Ma (Sossego ore body). The Archaean ages were discussed considering the hypotheses of the mineralization having or not a genetic link with the granitoids. In the first case, it could be related to the 2.76 – 2.74 Ga granitogenesis and the ages would represent the opening of the Pb isotopic system caused by subsequent thermal or deformational events. Alternatively, those ages could be related to the ~2.6 Ga magmatic event that is accounted for the dikes present at the neighboring Cu-Au 118 deposit, but its unexpressive significance in the Carajás region makes it unlikely to be responsible for the production of great cuprousauriferous deposits, like the Sossego. In the absence of a genetic link with granitoids, the mineralization could be related to metamorphic processes and the Pb-Pb ages would record the accompanying shearing event that caused the granitoid mylonitization and might have induced the infiltration of high salinity fluids, resulting in biotite-rich rocks with high Cl contents. No geological significance was given to the Mesoproterozoic age. A Sm-Nd isochron of 2578 ± 29 Ma was also obtained for ore samples. It is comparable to the Pb-Pb age of 2608 ± 25 Ma and most likely reflects an isotopic reequilibrium in response to later events. Model-ages (3.16 – 2.96 Ga) for the ore are similar to those of the deposit granitoid (3.12 – 2.98 Ga) and also to the Grão Pará Group basalts (2.76 – 3.09 Ga), suggesting that the ore could have been, at least in part, derived from these rocks. εNd values (-4.09 to -0.94) indicate an essentially crustal origin for both the host rocks and the ore. Based on the diagram εNd x t, it is plausible to admit that ore metals were derived from both granitoids and mafic rocks, especially the basalts from the Grão Pará Group. The δ 13CPDB values for calcite indicate a homogeneous source for C, probably mantle-derived. They show a negative linear correlation with the δ 18O values, suggesting a common evolution process. In terms of C and O natural reservoirs, the isotopic data are more consistent with sources of carbonatitic affiliation as indicated by the anomalous P and ETR contents found in the ore bodies. Calculation of δ 18Ofluid reveals that most values fall, respectively, in the ranges of -7.24 to -5.17‰ and +1.14‰ to +3.21‰ at 150° and 350ºC, being consistent with the participation of meteoric water in the Sossego hydrothermal system. However, higher values (+9.79‰ at 250ºC and +12.77‰ at 350ºC) suggest a magmatic or metamorphic water signature. Despite the speculations, integrating field, mineralogical, isotopic (stable and radiogenic) and fluid inclusion data allows to sustain that mineralization occurred at 2.76-2.60 Ga and was related to metamorphic-deformational processes, which triggered the migration of high saline fluids that had previously reacted with salt-rich sequences and became capable of transporting great amounts of metals, including Cu and Fe.
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Reconhecimento dos elementos deposicionais da lagoa do Violão na Serra Sul de Carajás: um estudo a partir de registros sismoestratigráficos
(Universidade Federal do Pará, 2016-11-21) ARAÚJO, Lidiane Cristina Lima de; SOUZA FILHO, Pedro Walfir Martins e; http://lattes.cnpq.br/3282736820907252
The sismoestratigraphy is one of the main tools used for the characterization of sequences and seismic facies, whose interpretation is based on the concepts of stratigraphic sequences. This study aims at the recognition and mapping of earthquake-depositional elements of Violão Lake - Serra Sul de Carajás - seeking to understand their sedimentary fill, considering the importance of lacustrine environments in the paleoenvironmental aspects, especially in tropical regions. The Violão Lake is part of the S11D plateau, built on lateritic ferruginous crusts at an altitude ~ 720 m above the sea level. It is a watershed of 1,836 km2, elongated in the NW-SE orientation, morphologically similar to a guitar. The research steps include acquisition, processing and interpretation of shallow seismic data. For the acquisition was used Bathy 2010PC ™ brand system SyQwest frequency 12 kHz and 3.5 kHz operating simultaneously with the aid of DGPS. They were collected 22 days profiles between 08/09 and 12/09/2014, treated in seismic software Reflex Win and Kingdom® Suite. 12 kHz profiles guaranteed greater quality information in relation to 3.5 kHz, and the six selected as the most representative elements earthquake-depositional patterns and ecocaráter. Stratigraphics three units were recognized (USI, USII, USIII), classified in chronological order from bottom to top. The US's rest on the rocky basement / acoustic that outcrop in the lake edges, extending to 25 m in the subsurface. Their morphology can be likened to a bowl of washing ("washing basin") of steep margins and relatively flat bottom. Their internal reflectors, sometimes discontinuous, made up of low to medium signal amplitude, unlike their neighboring surfaces. The USI is characterized by a progradational succession deposited from gravitational flow forming delta fronts for high lake conditions. The USII and USIII represent facies planeparallel, consistent with each other relatively soft reflectors. The recognition of 7 ecofacies allowed the relationship between the present acoustic signatures and the data of the distribution of superficial bottom sediments performed in previous works, which allowed an overview of the surface deposition of the Violão Lake, being the echoes of greater reflection related, predominantly, lamosos sediments with presence of peat and, in some moments, organic or oxidized sludge, favoring the occurrence of gases in the sedimentary interstices resulting from the decomposition of organic matter. In contrast, medium reflections low impedance contrast to refer to very fine sand with considerable mud content and good spread. Recognition and interpretation of stratigraphics standards identified different sedimentary filling processes of Violão Lake. The USI is characterized by a progradational succession of strata and, as they approach the depocenter, the strata of USII and USIII if have more uniform, as drapes in on a partially uneven surface under conditions of low energy influenced by flows shallow and intermediate, whose deposition occurs by suspension.