Isotopic domains controlled by transtensional and transpressional sectors in the auriferous Diadema shear belt, northern Brazil

Abstract

This paper presents carbon and oxygen isotope data from the auriferous Diadema shear belt, Carajás mineral province, northern Brazil. This area was affected by large-scale deformation accompanied by metamorphism and carbonate alteration of the metavolcanic rocks. Carbonate carbon-isotope data ( δ 13C PDB=−2.2 to −3.6‰, n=11) reveal that the hydrothermally altered zone within the shear zone has a different isotopic composition than that of the country rock (+0.4 to −1.7‰, n=12). Similarly, initial strontium isotope compositions reveal different isotopic compositions between the country rock of the shear zone ( 87Sr/ 86Sr=0.70454–0.70642) and the transpressure and transtensional zones ( 87Sr/ 86Sr=0.71286–0.71841), which indicates that the alteration zone resulted from large-scale fluid flow derived from an external reservoir. In addition to these differences, there are small variations in the δ 13C PDB values (from −2.8 to −2.0‰) in the shear zone, which may be related to preferential carbonate decomposition (and CO 2 devolatilization) in the transpressional domains. Evidence of carbonate decomposition is supported by calc-silicate rocks, which derived from the reaction between calcite and silicate minerals. The hydrothermal alteration and gold mineralization at the Diadema shear belt comprised both fluid infiltration from an external source and contemporaneous fluid generation in the shear zone. Compared with other gold deposits related to shear zones, the Diadema gold deposit developed at hypozonal crustal levels in the transition between greenschist and amphibolite metamorphic facies mineralogical assemblages.