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Browsing by Author "Costa, Mafalda"

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  • Mass spectrometric analysis of epimeric 3α/3β functionalized steroids.
    Publication . Ramos, Susana Sofia; Ramalho, Luís; Costa, Mafalda; Teixeira, Adriano; Almeida, Paulo; Costa, Maria do Céu
    Tandem mass spectrometry (MS/MS) using an atmospheric pressure chemical ionization (APCI) source in positive mode (+) reveals different intensity ratios of two fragment peaks of the epimeric cholestane derivatives synthesized from 3{460}- or 3β-cholestanol. The investigation of the fragmentation pattern obtained from their full scan and/or collision induced dissociation (CID) spectra in the case of the cholestane skeleton together with the respective electronic ionization (EI) spectra supports a distinction of the epimers.
    2007Journal article Open access Show more
  • Trace Element Geochemistry of Alluvial TiO2 Polymorphs as a Proxy for Sn and W Deposits
    Publication . Gaspar, Miguel; Grácio, Nuno; Salgueiro, Rute; Costa, Mafalda
    ABSTRACT: The Segura mining field, the easternmost segment of the Gois-Panasqueira-Segura tin-tungsten metallogenic belt (north-central Portugal), includes Sn-W quartz veins and Li-Sn aplite-pegmatites, which are believed to be genetically related to Variscan Granites. Sediment geochemistry indicates granite-related Ti-enrichments, locally disturbed by mineralization, suggesting magmatic and metamorphic/metasomatic titaniferous phases. Therefore, Segura alluvial samples and the geochemistry of their TiO2 polymorphs (rutile, anatase, and brookite) were investigated, and their potential as exploration tools for Sn and W deposits was evaluated. The heavy-mineral assemblages proved to be good proxies for bedrock geology, and TiO2 polymorph abundances were found to be suitable indicators of magmatic and/or metasomatic hydrothermal processes. The trace element geochemistry of Segura's alluvial rutile, anatase, and brookite is highly variable, implying multiple sources and a diversity of mineral-forming processes. The main compositional differences between TiO2 polymorphs are related to intrinsic (structural) factors, and to the P-T-X extrinsic parameters of their forming environments. Anomalous enrichments, up to 9% Nb, 6% Sn and W, 3% Fe, 2% Ta, and 1% V in rutile, and up to 1.8% Fe, 1.7% Ta, 1.2% Nb, 1.1% W 0.5% Sn and V in anatase, were registered. Brookite usually has low trace element content (<0.5%), except for Fe (similar to 1%). HFSE-rich and granitophile-rich rutile is most likely magmatic, forming in extremely differentiated melts, with Sn and W contents enabling the discrimination between Sn-dominant and W-dominant systems. Trace element geochemical distribution maps show pronounced negative Sn (rutile+anatase) and W (rutile) anomalies linked to hydrothermal cassiterite precipitation, as opposed to their hydrothermal alteration halos and to W-dominant cassiterite-free mineralized areas, where primary hydrothermal rutile shows enrichments similar to magmatic rutile. This contribution recognizes that trace element geochemistry of alluvial TiO2 polymorphs can be a robust, cost- and time-effective, exploration tool for Sn(W) and W(Sn) ore deposit systems.
    2022-10Journal article Open access Show more