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 Paran & PRIME;a 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 degrees 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 (Caic & PRIME;o 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 Paran & PRIME;a 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 Caico & PRIME; Complex during strike-slip motion on the Portalegre shear zone.

Automated summary

Shear zones in the Borborema Province of northeastern Brazil are efficient conduits for mineralizing fluids. The emerald occurrences in this region are associated with strike-slip deformation zones formed during the Neoproterozoic-Cambrian Brasiliano orogeny. The Portalegre shear zone, where the Paraná deposit is located, exhibits distinct aeromagnetic anomalies and mylonitic gneisses and schists with vertical foliation and horizontal mineral stretching lineation. The emerald mineralization is found in phlogopite and actinolite-phlogopite schists and is related to the interaction between Be-bearing pegmatite dykes and amphibolite lenses during strike-slip motion on the shear zone.