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- Inspecting zircon populations of the Iberian Pyrite Belt: tracking the Cadomian record of the South Portuguese ZonePublication . Lains Amaral, João; Solá, A. Rita; M. Bento dos Santos, TelmoABSTRACT: The palaeogeographic location of the southernmost zone of the Iberian Massif, the South Portuguese Zone (SPZ), prior to the amalgamation of Pangaea is still a matter of debate. In this work, we attempt to track its palaeogeographic setting during the final stages of the Cadomian Cycle.
- U-Pb zircon SHRIMP dating of a protracted magmatic setting and its volcanic emplacementPublication . Lains Amaral, João; Solá, A. Rita; Santos, Telmo M. Bento Dos; Tassinari, Colombo C. G.; Gonçalves, JoãoA geochronological study using SHRIMP U-Pb analysis of zircon grains has been conducted to date felsic volcanic rocks hosting the six massive sulphide deposits of the giant Aljustrel mining district in the Iberian Pyrite Belt. A multiple method age calculation approach was used to validate and ponder calculated Concordia ages (emplacement and inherited), which included weighted average, probability density peak(s), Tuff Zirc and Unmix functions. This approach was particularly useful to interpret the wide continuous single U-Pb ages (320–405 Ma) recorded in the Aljustrel volcanic rocks. The volcanic pile (>250 m) that hosts the Aljustrel deposits was emplaced between 359 and 353 Ma. Upper Devonian inheritance, representing subvolcanic activity, is well-represented in the volcanic rocks of Aljustrel (373–365 Ma). Older Devonian inherited zircon ages at 405 Ma, 388 Ma and 380 Ma were retrieved, hypothetically representing deep plutonism or other melting episodes, which suggests a long-lasting (~50 Ma) magmatic activity in the Aljustrel district. Older pre-Devonian inherited ages, uppermost Silurian and early to late Cambrian, and post-emplacement ages (~330–345 Ma) were also detected, with the latter reflecting Pb loss most likely driven by the main Variscan orogenic event. Maximum ages obtained for the volcanic rocks in the different deposits open the possibility that the last pulses of volcanic activity and subsequent deposition of the massive sulphides were diachronic in the different Aljustrel sub-basins. Additionally, results imply that, contrary to previously assumed, Gavião and São João-Moinho deposits are probably not the same ore lens disrupted by tardi-Variscan faults. This opens new opportunities for mining exploration and targeting in the Aljustrel district and points out the importance of high-resolution geochronological studies in mining and brownfield areas.
- The bimodal Fii-A2-type and calc-alkaline volcanic sequence of the Aljustrel brownfield region, Iberian Pyrite Belt, SW Iberian MassifPublication . Lains Amaral, João; Solá, A. Rita; Santos, Telmo M. Bento Dos; Feitoza, L. M.; Tassinari, Colombo C. G.; Crispim, Lourenço; Chichorro, Martim; Hofmann, Mandy; Gãertner, Jessica; Linnemann, Ulf; Gonçalves, JoãoABSTRACT: The Iberian Pyrite Belt (IPB) is a late Devonian- Early Carboniferous world-class polymetallic VMS province that includes significant Cu-(Sn)-Pb-Zn-(Ag) deposits of massive sulphides and feeder zones. The Aljustrel brownfield region contains one of the highest concentrations of ore in the IPB in 6 known deposits (Gaviao, Sao Joao, Moinho, Algares, Estacao and Feitais). To delve into the petrogenesis of the Aljustrel early Carboniferous (similar to 355 Ma) felsic-dominated bimodal volcanism, new whole-rock trace elements and Sm-Nd isotopes, and U-Pb in zircon were obtained. Based on Ga/Al and Y/Nb ratios, it is shown that Aljustrel felsic magmatism has the geochemical features of A2-type melts, typical of post-collisional and back-arc settings. U-Pb in zircon for a juvenile felsic volcanic rock point to antecrysts ages spanning from 387.9 to 366.6 Ma and a maximum emplacement age of 354.3 +/- 2.6 Ma. These long-lasting melting events, present in both juvenile (epsilon Ndi = +1.79) and evolved felsic rocks (epsilon Ndi =-5.07), imply heterogeneous sources dominated by zircon-bearing igneous rocks. The Sm-Nd model ages are in accordance with previous Lu-Hf model ages in zircon, reinforcing that the isotopic variability is related to the same petrogenetic process. Subordinated Aljustrel mafic rocks, coeval with the abundant felsic volcanism, show orogenic signatures, namely Nb-Ta-Ti negative anomalies and calc-alkaline affinities, whereas Sm-Nd isotopic data (epsilon Ndi = +1.54 to +5.48) points to variable to no contamination with crustal material. These geochemical results suggest derivation from an enriched mantle source modified by subduction metasomatism. In addition, the mafic rocks did not provide zircons for geochronological analysis, with the exception of one sample, in which a Concordia age of 402.1 +/- 15.5 Ma was obtained from a single grain. The combined geochemical signatures of mafic and felsic volcanic rocks suggest asthenospheric rise, but this solely does not explain the abundance of zircon antecrysts in the felsic rocks. Therefore, a geodynamic model that includes a continuous evolution from Devonian to Carboniferous times is inferred. This more complex and broader geodynamic model for the Iberian Pyrite Belt in which successive metal remobilization occurred after successive melting events, fits the present geochemical data and is more likely to explain why the Iberian Pyrite Belt is a unique metallogenetic province.
- Geology of the recently discovered massive and stockwork sulphide mineralization at Semblana, Rosa Magra and Monte Branco, Neves-Corvo mine region, Iberian Pyrite Belt, PortugalPublication . Pereira, Zélia; Matos, João Xavier; Solá, A. Rita; Batista, Maria Joao; Salgueiro, Rute; Rosa, Carlos J. P.; Albardeiro, Luis; Mendes, Márcia; Morais, Igor; De Oliveira, Daniel Pipa Soares; Pacheco, Nelson; Araújo, Vítor; Castelo-Branco, José Mário; Neto, Rodrigo; Lains Amaral, João; Inverno, Carlos; Oliveira, José TomásABSTRACT: The recently discovered massive and stockwork sulphide mineralization of Semblana-Rosa Magra and Monte Branco, situated ESE of the Neves–Corvo volcanogenic massive sulphide (VMS) deposit in the Iberian Pyrite Belt (IPB) is presented. Geological setting and tectonic model is discussed based on proxies such as palynostratigraphy and U–Pb zircon geochronology. The mineralization is found within the IPB Volcano-Sedimentary Complex (VSC) Lower sequence, which includes felsic volcanic rocks (rhyolites) with U–Pb ages in zircons of 359.6 ± 1.6 Ma, and black shales of the Neves Formation of late Strunian age. Massive sulphides are enveloped by these shales, implying that felsic volcanism, mineralization and shale sedimentation are essentially coeval. This circumstance is considered highly prospective, as it represents an important exploration vector to target VMS mineralization across the IPB, in areas where the Lower VSC sequence is present. The Upper VSC sequence, with siliciclastic and volcanogenic sedimentary rocks of middle–late Visean age, shows no massive mineralization but a late Tournaisian (350.9 ± 2.3 Ma) volcanism with disseminated sulphides was also identified. Nevertheless, stratigraphic palynological gaps were found within the Strunian and in the Tournaisian sediments, between the Lower and Upper VSC sequences, reflecting probable erosion and uplift mechanisms linked with extensional tectonics. The Semblana and Monte Branco deposits and the Rosa Magra stockwork are enclosed by tectonic sheets that dismembered the VSC sequence in a fold-and-thrust tectonic complex, characteristic of the NE Neves–Corvo region. The methodologies used allow a geological comparison between Neves–Corvo and other IPB mine regions such as Lousal–Caveira, Herrerias, Tharsis and Aznalcollar.
- Geology of the recently discovered massive and stockwork sulphide mineralization at Semblana, Rosa Magra and Monte Branco, Neves–Corvo mine region, Iberian Pyrite Belt, PortugalPublication . Pereira, Zélia; Matos, João Xavier; Solá, A. Rita; Batista, Maria Joao; Salgueiro, Rute; Rosa, Carlos; Albardeiro, Luis; Mendes, Márcia; Morais, Igor; de Oliveira, Daniel Pipa Soares; Pacheco, Nelson; Araújo, Vítor; Castelo Branco, José M.; Neto, Rodrigo; Lains Amaral, João; Inverno, Carlos; Oliveira, José T.Abstract The recently discovered massive and stockwork sulphide mineralization of Semblana-Rosa Magra and Monte Branco, situated ESE of the Neves–Corvo volcanogenic massive sulphide (VMS) deposit in the Iberian Pyrite Belt (IPB) is presented. Geological setting and tectonic model is discussed based on proxies such as palynostratigraphy and U–Pb zircon geochronology. The mineralization is found within the IPB Volcano-Sedimentary Complex (VSC) Lower sequence, which includes felsic volcanic rocks (rhyolites) with U–Pb ages in zircons of 359.6 ± 1.6 Ma, and black shales of the Neves Formation of late Strunian age. Massive sulphides are enveloped by these shales, implying that felsic volcanism, mineralization and shale sedimentation are essentially coeval. This circumstance is considered highly prospective, as it represents an important exploration vector to target VMS mineralization across the IPB, in areas where the Lower VSC sequence is present. The Upper VSC sequence, with siliciclastic and volcanogenic sedimentary rocks of middle–late Visean age, shows no massive mineralization but a late Tournaisian (350.9 ± 2.3 Ma) volcanism with disseminated sulphides was also identified. Nevertheless, stratigraphic palynological gaps were found within the Strunian and in the Tournaisian sediments, between the Lower and Upper VSC sequences, reflecting probable erosion and uplift mechanisms linked with extensional tectonics. The Semblana and Monte Branco deposits and the Rosa Magra stockwork are enclosed by tectonic sheets that dismembered the VSC sequence in a fold-and-thrust tectonic complex, characteristic of the NE Neves–Corvo region. The methodologies used allow a geological comparison between Neves–Corvo and other IPB mine regions such as Lousal–Caveira, Herrerias, Tharsis and Aznalcollar.