The Paleoproterozoic Rio Branco stock covers an area of ~31 km2 and is
intrusive in Archean calc-alkaline foliated biotite-bearing monzogranite of the
Canaã dos Carajás region, Carajás Mineral Province. It is constituted by
undeformed and isotropic, hololeucocratic syenogranite, showing equigranular
texture. The granite is formed essentially by perthite alkali feldspar, quartz, and
plagioclase, with additional variable amounts of chloritized biotite. Fluorite,
allanite, and zircon are accessory minerals, whereas magnetite, pyrite, and
chalcopyrite are scarce. Scanning electron microscopy studies indicate that
these rocks also contain Ce-fluocerite, xenotime, and heavy rare earth minerals.
Albitization and greisenization are the main alteration processes that affected
the granite, being the mineralogy represented by albite, fluorite, topaz, chlorite,
muscovite, siderophyllite, and iron oxides. Magnetic susceptibility values vary
between 1,3 x 10-5 a 6,9 x 10-4 (SI), which indicate the reduced character of the
granite. Geochemistry data on the Rio Branco Granite demonstrate its affinity
with reduced A2-type intraplate granite. It is subalkaline, metaluminous to
peraluminous and shows high FeOt/(FeOt+MgO). The rare earth element (REE)
patterns are flat with little heavy REE fractionation and show accentuated
negative Eu anomalies (Eu/Eu*=0,08-0,13). These characteristics are typical of
evolved granites, derived from volatile-enriched liquids responsible by the
intense subsolidus hydrothermal alteration. The dating of the Rio Branco granite
did not give conclusive results, due to the metamictic state of the zircon, but the
obtained data, associated with geological evidence, suggest that the granite is
Paleoproterozoic. The comparison between the Rio Branco granite and
Paleoproterozoic A-type suites of the Carajás Province suggests that the Rio
Branco granite presents greater affinity with the Velho Guilherme suite and, in
lesser degree, with the Serra dos Carajás suite. On the other hand, it is clearly
distinct of the oxidized A-type granites of the Jamon Suite. Two minor
occurrences of isotropic granites located near the Serra Dourada village and to
the north of the Planalto stock are also of syenogranitic composition.
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