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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