The large Carajás iron ores belongs to the Carajás fonnation, which is a 1 00-400m thick banded
and laminated iron formation Gaspilite ), 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.
Leveis ( 4 J.Ul1 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 (LREE=6,66; Eu*=3,54;
(La/Yb),=l,52) than the top (I:REE= 3,89; Eu*=3,18; (La/Yb),==,66). Besides, the major elements
content have more variability at base than top. The Carajás jaspilite have twice Ga (2lppm), 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 813C mean of -4.3%opoa and two groups of 8180 (+24,9 to +15,4 and +12,8 to
+6,6%osMow ). Otherwise, metamorphic imprints on this rocks are minimal.
Regional work, bibliographic compilation and correlations of the Carajás formation with
overlying units ofBahia and Azul mines leaves to propose a evolutionary model for the Grão Pará Basin,
initiated as a intracontinental rifting stage, marked by crusta! contaminated tholeiitic basalt volcanism
(2.76 Ga- U-Pb zircon ages). The second stage was the deposition ofthe Carajás formation over a wide,
quiet marine continental she1f, in:fluenced by upwelling of Fe-Si rich waters. In a third stage, the last was
recovered by volcanics associated with clastic 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 closes the
evolution.
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