Structural and geochronological constraints on the Portalegre shear zone: Implications for emerald mineralization in the Borborema Province, Brazil

Shear zones serve as efficient conduits for driving mineralizing fluids within the crust. In the Borborema Province, northeastern Brazil, emerald occurrences are associated with zones of strike-slip deformation that formed during the final stages of the Neoproterozoic-Cambrian Brasiliano orogeny. We present aeromagnetic, structural, and geochronologic data across the Portalegre shear zone to understand emerald formation at the Paraná deposit. In this region, shear zones are marked by conspicuous aeromagnetic anomalies that coincide with mapped mylonitic gneisses and schists showing a vertical to sub-vertical foliation and horizontal mineral stretching lineation. Dextral motion of the shear zone is indicated by deformed asymmetric augen, pegmatite boudins, and S–C and S–C–C’ fabrics. Emerald is found in phlogopite and actinolite-phlogopite schists, and prismatic crystals occur parallel to the schist foliation plane. Deformation temperatures in the range of 390–530 °C are supported by microstructural analysis as evidenced by quartz recrystallization controlled by subgrain rotation. Zircon U–Pb ages of veins and boudins of granitic composition interleaved with the schist yielded isotopic analyses of ca. 2.20–2.21 Ga, suggesting intense reworking of the orthogneissic basement (Caicó Complex) during ore formation. Phlogopite ⁴⁰Ar/³⁹Ar analysis yielded ages of ca. 524–528 Ma for the mineralized phlogopite schist, the best estimation for the emerald mineralization at the Paraná deposit. Integration of available data implies a tectonic magmatic-related origin for the deposit, involving the formation of the emerald-bearing phlogopite schists after metasomatic interactions between Cambrian Be-bearing pegmatite dykes and amphibolite lenses of the Caicó Complex during strike-slip motion on the Portalegre shear zone.