O ambiente deposicional da formação Carajas e uma proposta de modelo evolutivo para a Bacia Grão Para

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

The large Carajás iron ores belongs to the Carajás formation, which is a 100-400m thick banded and laminated iron formation (jaspilite), located at Pará state in North Brazil. This almost continuous formation outcrops for at least 260km, in 60 ore deposits, distributed in three main ridges, São Felix, Leste and Carajás. The last one is a sinformal structure sub-divided in South and North ridges. This work was carried out on detailed mapping, stratigraphic raising and petrographic, geochemical, isotopic and geochronological sampling of the North ridge, where the mining activity is currently running and bench and drill-core are available. Levels (4 ?m to 3 cm) composed by chert or jasper alternated with magnetite-maghemite-hematite was deposed between 2,754 and 2,744 Ma (22m/Ma) at depths of 100-200m, locally affected by bottom currents. This hydroplastic chemical sediments precipitated by supersaturation (Si) and oxidation (Fe) from upwelling waters where the base was richer in hydrothermal source waters (?REE=6,66; Eu*=3,54; (La/Yb)N=I,52 ) than the top (?REE= 3,89; Eu*=3,18; (La/Yb) N =O,66). Besides, the major elements content have more variability at base than topo The Carajás jaspilite have twice Ga (21ppm), Bi (6ppm), Pb (18ppm) and seven times Sb (7ppm) than the world average for similar rocks. The Fe oxidation may have be promoted by organic activity, attested by delicate double wall spherulites and kerogen preservation in siltstones of a light younger unit. Local hydrothermal carbonatization has affected the jaspilite producing ?13C mean of -4.3?PDB and two groups of ?18O (+24,9 to +15,4 and +12,8 to +6,6?SMOW). Otherwise, metamorphic imprints on this rocks are minimal. Regional work, bibliographic compilation and correlations of the Carajás formation with overlying units of Bahia and Azul mines leaves to propose a evolutionary model for the Grão Pará Basin, initiated as a intracontinental rifting stage, marked by crustal contaminated tholeiitic basalt volcanism (2.76 Ga - Pb-Pb zircon ages). The second stage was the deposition of the Carajás formation over a wide, quiet marine continental shelf, influenced by upwelling of Fe-Si rich waters. In a third stage, the last was recovered by volcanics associated with dastic sedimentation (2.74 Ga - Pb-Pb zircon ages). The fourth stage comprises the installation of another continental shelf environment, where clastics and carbonate rocks has deposed (2.68 Ga - U-Pb zircon ages). Basin inversion and fluvial deposition doses the evolution

ASSUNTO(S)

serra dos (pa) rochas metamorforicas - carajas geoquimica - carajas serra dos (pa) serra dos (pa) serra dos (pa) geologia isotopica - carajas ferro minas e mineração - carajas

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