Telescoped transition from trellis to silicate-mottled magnetite records deep shear metamorphism in the IOCG deposit at Jatobá, Carajás, Brazil

Abstract Jatobá is a magnetite-hosted Cu-Ni deposit in the Carajás Domain, Brazil. The deposit is located along E–W structures belonging to the Canaã dos Carajás shear zone and hosted within the Neoarchean Grão Pará Group. Micron to nanoscale investigation of magnetite from mafic host lithologies and Cu-Ni-mineralisation facilitate constraints on ore genesis. Two texturally and geochemically distinct types of magnetite are defined: ‘trellis’ (country rocks) and ‘silicate-mottled’ (ore). An overlap between the types is recognised as ilmenite changes from lamellar trellis to blebby and patchy textures in the silicate-mottled magnetite. The blebby type comprises Al-hydroxides (gibbsite) and talc, replacing hercynite and Mg-amphibole, respectively. The mottled magnetite contains Mg-(Fe)- and Ce-bearing calcic amphiboles, both associated with non-classical pyriboles (NCP). Geochemical signatures change from Ti-Cr-Co-Mn in trellis magnetite to a pronounced REE enrichment in the mottled type. Nano-inclusions of allanite occur as epitactic intergrowths with actinolite within magnetite. Amphiboles in the host rocks mirror those found in magnetite, with ferro-tschermakite present in both cases. Ilmenite-magnetite nano-thermobarometry yields a range of temperature and log f O 2 values (temperature from 728°C at log f O 2 = –12 to 414°C at log f O 2 = –31) for re-equilibration between magnetite and ilmenite from initial trellis to the trellis + blebby and to patchy ilmenite in the densely mottled magnetite. Ferro-tschermakite geobarometry enables an estimate of 6.4–7.4 kbar, compatible with amphibolite-facies metamorphism at ∼20 km depth. Syn-metamorphic deformation textures include magnetite + apatite as pods, banding and folds, as well as sigmoidal scapolite and pressure shadows surrounding magnetite. Collectively, these data support a genetic model implying deep shear-zone metamorphism at the base of the Canaã dos Carajás strike-slip structure. This area is a reservoir for metal sources as fluids can tap into granitoids and ultramafic lithologies in the basement. The telescoped transition from trellis to silicate-mottled magnetite records fluid fluxes of variable overprinting effects during protracted fluid–rock interaction.