Journal of the Geological Survey of Brazil

Volcanogenic features of IOCG deposits of North Carajás, Brazil, and related constraining evidence

Sergio Martini, Ana Maria Dreher — Thu, 14 Mar 2024 00:00:00 +0000

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.

A field guide to the sand-rich turbidite systems of the Taciba Formation, Itararé Group, Paraná basin, Brazil

Fri, 09 Feb 2024 00:00:00 +0000

This field guide provides a detailed outcrop logistics, access and description of the sand-rich turbidite deposits of the Taciba Formation (Rio do Segredo Member), the uppermost unit of the Pennsylvanian Itararé Group (Paraná Basin), which crops out in the Santa Catarina State, southern Brazil. This area has become a classic destination for professional’ field courses and students interested in outcrop analogous to offshore turbidite reservoirs. Although some of the outcrops have been widely described, some localities remain unexplored. These turbidite systems are here subdivided on the basis of their degree of confinement, relative to the Precambrian basin floor topography. The bedrock morphology was shaped during the Late Paleozoic Ice Age. Advance and retreat phases of glaciers are represented by superficial scouring into the crystalline basement. This glacially-carved topography in turn controlled the subsequent sedimentation by forming a basal nonconformity in the Taciba Formation. Thus, the turbidity currents and their deposits, exposed in three main areas along the eastern margin of the Paraná basin, were classified as confined and/or unconfined. This field guide is also valuable for nourishing the ongoing debate on the turbidity currents interpretations based on sedimentological descriptions of the main localities and their major features. Field guides are quite popular in North America and Europe, among other places, but not so popular in Brazilian literature. The present contribution aims to bring details on some little known turbidite-outcropping localities of the Taciba Formation. It also intends, through its format, to attract interest of national and international geologists on Brazilian key outcrops and offer a synthesis and a starting point for teaching purposes and future studies.

Investigation of Archean gamma-ray fingerprint: Methodology and tectonic application in central Brazil

Mon, 08 Jan 2024 00:00:00 +0000

Gamma-ray spectrometric responses of Archean rocks were studied in order to define a characteristic signature for these rocks. As the half-life of the most frequent Thorium isotope (²³²Th – half life of 14.05 Ga) is more than 3 times longer relative to the Uranium isotope (²³⁸U – half life of 4.50 Ga), a low response of radiation intensity in the spectral range of decay series of Uranium relative to that of Thorium is expected for Archean rocks. Based on this theoretical aspect, associated with gamma-ray spectrometric responses parametrically studied in several Archean areas of Brazil, gamma-ray and magnetometric structural interpretation, follow-up field studies, micropetrography, lithogeochemistry, and geochronology (U-Pb in zircon), it was possible to define and identify previously unidentified Archean rocks (2.82 Ga) tectonically imbricated amid Neoproterozoic magmatic arc terrains. This discovery has geotectonic implications regarding the magnitude of the Transbrasiliano Lineamento and its potential to fragment large Archean blocks and arrange them aligned within the Tocantins Province, Central Brazil.

Inference of minerals at Redondão kimberlite crater, Parnaíba Basin, Brazil, based on interpretation of airborne gamma-ray spectrometry and ASTER data

Adolfo Silva — Mon, 08 Jan 2024 00:00:00 +0000

This work reports how minerals at Redondão kimberlite pipe may be inferred through integrated interpretation of airborne gamma-ray spectrometry data and multispectral images derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Using several processing techniques (e.g. K, eTh, and eU concentration maps, K/eTh, K/eU, and eU/eTh ratio maps, K image inverse map, profile analysis for radiometric data and; spectral indexes and mineral distribution and abundance maps for ASTER data), it was observed that K concentrations within Redondão kimberlite intrusion are high enough to be distinct from the surroundings and at least three spectral signatures of endmembers extracted from ASTER images suggest that chlorite, diopside, and phlogopite minerals may be present within Redondão kimberlite crater and the near surroundings. Although the sources of K concentrations are unclear, it probably is composed of different minerals and phlogopite may be one of them. The spectral signatures of chlorite, diopside, phlogopite may be derived from clastic materials and products derived from ultramafic mineral weathered, but new studies are recommended to improve distribution and abundance maps of these minerals. These new studies may include hyperspectral sensor images and rock-spectra data. The results reported here be useful for further research whose goal is to map kimberlites using remotely sensed images not only in the study area, but also in other locations with similar geologic context.

Gamma-ray spectrometry, magnetic and gravity signatures of Archean nuclei of the Borborema Province, Northeastern Brazil

Tue, 20 Feb 2024 00:00:00 +0000

Gamma-ray spectrometry, as well as magnetic and gravity data, are used to investigate the geophysical signatures of the Archean nuclei of the Borborema Province. Natural radioactivity, magnetic anomalies, and residual Bouguer gravity anomalies of the Archean nuclei exhibit distinct signatures in relation to adjacent Proterozoic domains. Gamma-ray spectrometry data reveal eTh enrichment in relation to K and eU contents in Archean units. Assuming that K and U were the dominant isotopes, this relative enrichment of eTh can be explained by the fact that Th radioisotopes have a longer half-life than the other two radionuclides and that 4.56 Ga has elapsed since Earth's formation. The intensity of the total magnetic gradient in Archean units is greater than in Proterozoic units in most nuclei. The Archean units underwent deformation and metamorphism in the Brasiliano/Pan-African Orogeny; therefore, the magnetic characteristics now observed in Archean mafic-ultramafic rocks, iron formations and gneiss-migmatite complexes are the joint result of their primary properties and the superposed effects of the orogeny. All Archean nuclei of the Borborema Province show positive residual Bouguer gravity anomalies. This could be due to the conservation of the main petrophysical properties of the Archean lithosphere, and their preservation during the intense granitization that occurred in the Brasiliano/Pan-African Orogeny. As magnetic and gravity methods provide information from depth, it is possible to infer the continuity of some Archean nuclei beyond the limits established by surface geological data. Based on these results, it will be possible to use geophysical signatures to investigate the possible existence of unknown Archean units in the province.