Abstract
The granitic rocks that host gold mineralizations in the Batalha gold field (Pará State, Brazil) present granophyric structures that indicate crystallization at shallow levels and the influence of alkalis- and silica-rich, post-magmatic fluids. Locally, they present weakly developed rapakivi textures and their composition is predominantly of 3a and 3b granites. Their petrographic characteristics allow correlation with the Maloquinha Suite rocks, of ages around 1.84 Ga. Petrographic studies carried out using samples from drill-holes that intersect zones mineralized in gold made the characterization of ample pervasive hydrothermal alteration zones possible, showing continuous variations of the fluid compositions from the post-magmatic stages. The oldest event is characterized by sodic alterations, followed by potassic alteration producing microcline and biotite, which confers a dark red color to the rocks; propylitic alteration is superimposed on both. Final stages may have evolved to locally pervasive sericitic alteration, but this type was better characterized as of fissure style and associated with minor shear zones. Gold mineralizations are predominantly associated with propylitic alterations, but the highest grades are observed in sericitic zones and, more specifically, as free, coarse-grained gold hosted by minor quartz veins, with associated carbonates, sulfides and fluorite.
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