The Antonio Vicente Granite (AVG), located in the northwest of Sao Felix do Xingu,
Carajás Province, is one of the bodies of the Velho Guilherme Intrusive Suite. It consists of
isotropic syenogranite to monzogranite hololeucocratic to leucocratic rocks altered in
different intensities by late to post-magmatic processes. The AVG hosts mineralization of Sn
and other metals (Ta, Nb, Zr, Y) in its more evolved rocks and associated bodies of
greisens. Geochemically, presents metaluminous to weakly peraluminous character, signature
of A-type granites, and geotectonic affinity with intraplate granites. Four major petrographic
domains were recognized: (1) Biotite-amphibole monzogranite to syenogranite (BASMG),
preserved of late to post-magmatic alterations, (2) Amphibole-biotite syenogranite (ABSG),
(3) Biotite monzogranite (BMG) and , (4) Biotite syenogranite (BSG) with variations to types
altered and intensely altered, called, respectively, Altered biotite syenogranite (ABSG) and
Intensely altered biotite syenogranite (IABSG). Tabular bodies of greisens occur in fractures
and faults into syenogranite rocks. Petrographic and geochemical studies show that fractional
crystallization was the major petrogenetic process that governed the evolution of this body.
Five types of quartz were identified based on morphological and textural analysis using SEM CL: Qz1, luminescent, anhedral, fractured, medium to-coarse grained, locally zoned, present
in all facies and considered the earlier type. Qz2, little luminescent, it forms irregular patches
or discontinuous gray to dark gray edges on the Qz1; it is present in greater intensity in the
more evolved rocks. Qz3, considered the later type, show black color and fills microfractures,
sectioning previous types. It is rarer in greisens associated with AVG. Qz4, identified in the
IABSG rocks and greisens, it forms well developed anhedral crystals, slightly fractured and
with variable luminescence. It seems to mark the transition from magmatic to hydrothermal
stage. Qz5, it was identified only in bodies of greisens or filling cavities in IABSG. It appears
as euhedral crystals slightly fractured, sometimes forming clusters of fine crystals. It shows
light-dark zoning in a typically well-defined hydrothermal pattern. Crystals of cassiterite are
commonly associated with Qz4 and Qz5 types. The morphological and textural features
suggest that magmatic quartz (Qz1) was modified and transformed into Qz2 and Qz3 by
hydrothermal fluids that percolated the AVG. These alterations are more intense in the ABSG
and IABSG. In the greisens, quartz crystals show typically hydrothermal quartz pattern.
Preliminary chemical analysis by LA-ICP-MS on quartz crystals showed significant changes
mainly in the contents of Ti, Al and Ge. The continuous decrease in the content of Ti with the
differentiation of the AVG confirmed the compatible behavior of this element in granitic rocks and showed a negative correlation with Rb/Sr whole rock. Al showed a more irregular
behavior, but decreased in the same direction. The Ge, considered a good indicator of
magmatic differentiation, presented constant Ge/Ti ratios in the syenogranite and
monzogranite rocks, but higher in intensely altered rocks and in the greisen, been a good
indicator of magmatic evolution in granitic systems. The study of SEM-CL showed that
quartz was an excellent marker of the changes imposed by the late to post-magmatic process
that acted in the AVG. This study opens a new line of research in the GI of the UFPA, making
this an important tool in geological studies.
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