Fault zone fluid pathways in the carbonate Irecê basin, NE Brazil

We applied field structural data and isotope geochemical (δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr) analyses to understand the relationship among calcite veins, fault damage zones and carbonate host rocks in a thrust fault damage zone in the Achado quarry, Irecê Basin in the São Francisco Craton, NE Brazil. Our results reveal three hydrological packages with different rheological behaviours in a stratified carbonate succession. The upper package includes the Achado fault damage zone that is characterised by interlayered dolomitised grainstones and mudstones. These rocks display high positive δ¹³C values (10‰–13‰), negative δ¹⁸O values (mean −6.34‰) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotope values (0.70885–0.71519). A second package is marked by a cataclastic brittle shear zone lateral parallel to dolograinstones bedding. These rocks show low to positive δ¹³C values (−3.41‰ to +8.85‰), more positive δ¹⁸O values (mean −3.73‰) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotope values (0.71039–0.71373). The lower package is characterised by well-preserved pristine limestone succession that shows δ¹³C values ranging between −0.46‰ and +3.17‰, mean δ¹⁸O = −5.41‰ and less radiogenic ⁸⁷Sr/⁸⁶Sr values (0.70762–0.70818). In contrast to the upper and intermediate packages, rocks from the lower one exhibit very low permeability and behaved as a seal for fluid migration. Fluid flow occurred several times during basin evolution, for example along syn-rift fault damage zones, bedding-parallel carbonate breccia, thrust faults, cataclastic shear zones, synorogenic conjugate shear fractures or joints and opening mode I fracture-fill calcite veins. These fractures allowed pervasive fluid flow in the porous intermediate cataclastic shear zone where fluids flowed and formed veins, as diffuse fluid flow in randomly oriented fracture swarms, or channelised fluid flow in aligned fracture corridors. They record significant centimetre-scale to km-scale hydrological behaviour within carbonate layers. Most carbonates that are associated with veins, fault damage zones and hydraulic breccia were formed by fluids of the same origin with low δ¹³C (−6.0 to −2.0‰) and δ¹⁸O (−6.0 to −8.5‰) values, and more radiogenic ⁸⁷Sr/⁸⁶Sr values compared to the carbonate host rocks.