Paragenesis, geochemistry and metallogeny of the Dublin Gulch intrusion-related Au deposit, Yukon Territory, Canada

This study concerns the Dublin Gulch intrusion-related gold system, Yukon Territory, Canada. Located 85 km, north northwest of the town of Mayo, YT, the property hosts a 3.3 M oz Au deposit associated with a mid-Cretaceous pluton. A detailed, 8 stage, hydrothermal paragenesis has been constructed for the deposit. At least two discrete fluids are responsible for sulfide mineralization at Dublin Gulch. The latter of the two hydrothermal fluids is responsible for the majority of Au endowment on the property. Geochemical signatures of mineralization support this finding, displaying distinct populations of arsenopyrite compositions and sulfur isotopes for each fluid. Lead isotopes from sulfosalts associated with the second fluid suggest hydrothermal scavenging from country rocks. Geochronology and petrogenetic studies show that a short lived intrusive event c. 93-94 Ma took place at Dublin Gulch and that the main Au mineralising fluid may be linked to a yet unseen intrusion at depth.

Metalogenia em terrenos de alto grau metamórfico: as mineralizações tungstaníferas da área de Monumento/Cacaria RJ

Em trabalho de prospecção geoquímica regional por concentrado de bateia (PEREIRA e SANTOS, 1983) foi evidenciado na região de Cacaria, município de Piraí, ocorrências de scheelita. Com o objetivo de investigar a possível fonte litológica desta ocorrência, foram realizadas amostragens de sedimentos de corrente e concentrado de bateia além de investigações de campo para sua fonte. Elaborouse ainda um mapa geológico em escala de detalhe 1:25.000. As unidades estratigráficas base utilizadas no mapa foram definidas em trabalhos da empresa RIOFINEX (1977) durante pesquisas para mineralizações de Pb-Zn na região de Rio Claro. Ensaios laboratoriais envolvendo descrição de lâminas petrográficas, difratometria de raios X, fluorescência de raios X, espectrometria Raman, microscopia eletrônica de varredura e o auxílio do mineralight de ondas curtas. As mineralizações scheelitíferas encontradas foram classificadas segundo um modelo do tipo sedimentar exalativo, localmente com possíveis contribuições de atividades metassomáticas tardias, concentrando a mineralização, assim como proposto para algumas ocorrências já descritas no nordeste brasileiro.

Geochemical and radioactivity characterization of rocks from the Rio Preto (Go) Project

The Rio Preto Project, developed by the extinct Brazilian nuclear state company, Nuclebrás, during the late 70s and early 80s, consisted of basic geological mapping and radiometric characterization by aerogeophysical gamma-ray spectrometry, without channel discrimination, of a surface area of 650 km2 located to the west of the Chapada dos Veadeiros National Park on the northeastern of Goiás State, Brazil, including the confluence area of Claro and Preto Rivers. Additionally, the natural radioelements U, Th and 40K were determined by gamma-ray spectrometry in 300 rock samples from cores of the Rio Preto Project area. The tests were conducted at LABIDRO-Isotopes and Hydrochemistry Laboratory of the Departament of Petrology and Metallogeny (DPM) of the Institute of Geosciences and Exact Sciences, UNESP, in Rio Claro, SP, Brazil. This paper reports the results of petrographic characterization and chemical analyses of major oxides (SiO2, TiO2, Al2O3, Fe2O3, MgO, MnO, K2O, Na2O, CaO and P2O5) for all samples used to determine the natural radioelements present in the region. The organic matter content results obtained by colorimetry are also reported for selected cores of different lithotypes in order to investigate the possible relationship between graphite and the radioelements uranium and thorium. Finally, uranium content and 234U/238U activity ratio data for selected samples of schists and gneisses of the Lower Member of the Ticunzal Formation suggest the influence of weathering processes in the area. © 2012 Sociedade Brasileira de Geofísica.

Caracterização da radioatividade das águas do aquífero Guarani

This research aimed to evaluate the quality of “Aquifer Guarani’s” waters in the context of the current legislation related to the radioactive aspects, besides correlating the values obtained with chemical data already available, seeking to make an integrated analysis of the data in order to establish possible tendencies with directions of underground flow. The determinations of the total radioactivity beta and alpha were accomplished through the employment of the available infrastructure at the LABIDRO – Laboratory of Isotope and Hydrochemistry of the Petrology and Metallogeny of UNESP’s Institute of Geosciences and Exact Sciences, largely implanted through resources supplied by FAPESP and CNPq. The systems were gauged for the determinations of interest with tracer of known activity. Preliminaries tests were accomplished with high radioactivity samples of water to check the applicability of the methods. It was ended that the waters analyzed possess low radioactivity, as much alpha as total beta, and, therefore, they are appropriate for the human consumption; that there is not lineal relationship between the obtained data and the chemical data already available; and that the direction of the underground flow of the Aquifer doesn't check any tendency to the generated data.

Geologia do platô N6 - província mineral de Carajás (SP)

The N6 Plateau presents an iron-ore occurence in Carajás Mineral Province, standing near to actually operating deposits. Geological mapping in 1:10,000 scale and integration of geochemical, geophysical, petrography and drilling turns possible interpretation of his geological evolution. The mapped area has lithotypes from Archean Grão Pará Group, comprising very lowgrade metamorphic basic rocks and iron formation and an Proterozoic sedimentary association of conglomeratic sandstones called as Caninana Unity. The structural geology in given by a regional scale homoclinal, where the Grão Pará Group strata dips towards SW, as a part of the Northern Limb of the Carajás Fold. Subsequent deformation associated to the installation of the Carajás Shear Zone presents as E-W fold axis. Geochemical evidence permits to consider de Parauapebas Formation as the rocks which has been hydrothermally-altered to outsourcing fluids responsible to deposition of iron formations in the oceanic system, including different signatures which can be interpreted as possible sub-embayments in the Carajás Basin. The iron ore in the area occurs in subsurface as very fine friable hematite generated by supergenous enrichment of the iron formation. The conceived geologic model differs from the current academic proposal on the fact that hydrothermal alteration has been involved on the jaspelite enrichment. Metamorphism on the Parauapebas Formation presents paragenesis considered as ocean-floor metamorphism which precedes de deformation insofar as the rocks show no tectonic fabric referring to shallow crust evolution. Geophysical methods such as magnetometry and gravimetry presents excellent results for structural interpretation in uneven exposed terrain