Volcanogenic features of IOCG deposits od North Carajás, Brazil, and related constraining evidence
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Abstract
The magnetitic Cu-Au (IOCG) deposits of north Carajás, Amazonian Craton, Brazil, are hosted in the Neoarchean (2.76 Ga), dominantly basaltic-greywacke Itacaiúnas metavolcanic-sedimentary sequence that evolved in a continental rift setting. The mainstream interpretation of these deposits is that they are epigenetic, though they do show common features observed in syngenetic deposits of volcanogenic affiliation. The features include hosting tectonic and geological settings with coeval granitic and mafic heat sources; evidence of syndepositional faulting, exemplified by intraformational fragmental rocks; attending hydrothermal alteration represented by extensive Na-Ca, spilitic and chloritic alteration along with deposit-scale magnesian (chlorite, dalmatianite), potassic (biotite) and aluminous (andalusite, sillimanite) products; strata-bound morphology, with ore hosted in banded oxide-silicate facies iron-formations and debris flow breccia bodies located at eruptive breaks, where quiescent waning volcanic conditions favorable to ore deposition prevail; and near-deposit exhalites, for instance, oxide iron-formation, metachert and laminated quartz-amphibole-tourmaline-garnet rocks, that may extend laterally away from the deposits constituting ore-equivalent horizons. This altogether suggests that the deposits are intrinsic components of the stratigraphic package they are hosted in and constitute syngenetic hydrothermal products. Regarding constraining evidence – as represented by fluid inclusion, stable isotope and geochronological data – the available information does not rule out syngenesis. Fluid inclusions reveal highly saline mineralizing fluids that explain the low sulfur content of ore. Sulfide sulfur signature is near zero per mil, usually found in Archean volcanogenic deposits. Available stable C, O, H and B isotopic data can to a good extent be explained in connection with syngenetic systems, with particular reference to the evaporitic boron isotopic signature of tourmaline constituting firm argument against granite-related epigenesis, whereas evidence of metamorphogenesis is exiguous. Pb-Pb ages on sulfides usually coincide with the age of the volcanic-sedimentary host unit, whereas supposedly robust epigenesis supporting ages have been considered doubtful due to, for example, problematic dating of granitic sources and/or geological inadequacy through contradicting dated contact relationships. Considering this scenario, we maintain here our former position that the scrutinized deposits represent synvolcanic IOCG hydrothermal systems. Metamorphism- and deformation-related complexity mar the unraveling of this metallogenic connection, blurring detail features and nourishing misinterpretation. The large-scale geological integrity of the deposits, however, has been greatly preserved, as per the coherent stratigraphic distribution, strata-bound morphology and typological attributes. Unexpectedly, they consist of disseminated sulfides, with magnetite taking the place of the usual pyrite of VMS deposits, a peculiar aspect that is nonetheless shared by other volcanogenic deposits.
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