Tracing the geodymamic evolution of the North Gondwana

Pliocene–Pleistocene sand of the Alvalade basin was sampled from the sea-cliffs of SW Iberia close to Cape Sines for a provenance study using LA-ICPMS U-Pb dating of detrital zircons. The results are used to compare age distributions and to trace potential source areas based on existing knowledge of zircon-forming events in the pre-Cenozoic basement of SW Iberia. The 492 U-Pb ages obtained span a wide interval ranging from Cretaceous to Archean, with predominance of Paleozoic (31–71%; mainly Carboniferous), Neoproterozoic (19–20%; mainly Cryogenian–Ediacaran) and Cretaceous (21–39%). Two important features were noted: i) Carboniferous ages younger than ~ 315 Ma infrequent or absent in SW Iberia, and ii) Cretaceous ages are interpreted to indicate a Sines massif provenance. The data obtained confirm previous studies that indicate a sediment source in the Paleozoic basement of SW Iberia (Ossa-Morena and South Portuguese zones) but also suggest a more complex history of drainage than previously documented, involving other sources located to the north. The detrital zircons age populations also enable recognition of previously undocumented sources of Pliocene–Pleistocene sands and decipher their paleotectonic meaning: i) Neoproterozoic to Early Paleozoic meta-sedimentary and meta-igneous rocks of the Ossa-Morena and Central- Iberian zones formed in North Gondwana during the Cadomian orogeny and opening of the Rheic Ocean, and ii) Devonian to Carboniferous sedimentary and igneous rocks of the Ossa- Morena, Central Iberian, and South Portuguese zones formed when Gondwana and Laurussia collided (Variscan orogeny)

Provenance study of Pliocene–Pleistocene sands based on ancient detrital zircons (Alvalade basin, SW Iberian Atlantic Coast)

Pliocene–Pleistocene sand of the Alvalade basinwas taken fromthe sea-cliffs of SWIberia coast for a provenance study using radiometric dating. The U–Pb ages obtained revealed a wide interval ranging from Cretaceous to Archean, with predominance of Paleozoic, Neoproterozoic and Cretaceous zircon ages. Cretaceous ages interpreted to indicate a Sines Massif provenance are dominant in sands close to Cape Sines but are absent in sand sampled 12 km north. Carboniferous ages younger than ca. 315 Ma suggesting a possible contribution from the Central-Iberian Zone originally; however, these zircons may be multi-cyclic, having been reworked from Eocene–Miocene siliciclastic deposits related to transport from central Iberia (Lower Tagus basin drainage evolution). These signatures provide important constraints on the location and extent of the Pliocene–Pleistocene topography and drainage system that were probably controlled by: i) Miocene to Pleistocene landscape rejuvenation driven by Alpine movements along major faults; and ii) residual reliefs related to inherited Variscan structure. The U–Pb ages obtained also trace the pre-Cenozoic paleotectonic evolution of SWIberia recorded in their sources: i) the North Gondwana accretion and breakup; ii) the Gondwana and Laurussia collision; and iii) the Pangea breakup and opening of the Atlantic Ocean

Provenance of Holocene beach sand in the Western Iberian

Detrital zircons from Holocene beach sand and igneous zircons from the Cretaceous syenite forming Cape Sines (Western Iberian margin) were dated using laser ablation – inductively coupled plasma – mass spectrometry. The U–Pb ages obtained were used for comparison with previous radiometric data from Carboniferous greywacke, Pliocene–Pleistocene sand and Cretaceous syenite forming the sea cliff at Cape Sines and the contiguous coast. New U–Pb dating of igneous morphologically simple and complex zircons from the syenite of the Sines pluton suggests that the history of zircon crystallization was more extensive (ca 87 to 74 Ma), in contrast to the findings of previous geochronology studies (ca 76 to 74 Ma). The U–Pb ages obtained in Holocene sand revealed a wide interval, ranging from the Cretaceous to the Archean, with predominance of Cretaceous (37%), Palaeozoic (35%) and Neoproterozoic (19%) detrital-zircon ages. The paucity of round to subrounded grains seems to indicate a short transportation history for most of the Cretaceous zircons (ca 95 to 73 Ma) which are more abundant in the beach sand that was sampled south of Cape Sines. Comparative analysis using the Kolmogorov–Smirnov statistical method, analysing sub-populations separately, suggests that the zircon populations of the Carboniferous and Cretaceous rocks forming the sea cliff were reproduced faithfully in Quaternary sand, indicating sediment recycling. The similarity of the pre- Cretaceous ages (>ca 280 Ma) of detrital zircons found in Holocene sand, as compared with Carboniferous greywacke and Pliocene–Pleistocene sand, provides support for the hypothesis that detritus was reworked into the beach from older sedimentary rocks exposed along the sea cliff. The largest percentage of Cretaceous zircons (<ca 95 Ma) found in Holocene sand, as com- pared with Pliocene–Pleistocene sand (secondary recycled source), suggests that the Sines pluton was the one of the primary sources that became progressively more exposed to erosion during Quaternary uplift. This work highlights the application of the Kolmogorov–Smirnov method in compar- ison of zircon age populations used to identify provenance and sediment recycling in modern and ancient detrital sedimentary sequences

Sedimentary record of the amalgamation and break-up of Pangaea: U-Pb detrital zircon geochronology and provenance of Carboniferous-Triassic siliciclastic rocks (SW Iberia)

In SW Iberia there is a basal unconformity where Triassic sandstones overlie previously deformed Carboniferous turbidites. This important stratigraphic boundary records the transition from the final stages of Pangaea’s amalgamation to the initial stages of break-up. U-Pb dating of detrital zircon from Carboniferous greywackes of the South Portuguese Zone and Triassic sandstones of the Algarve and Alentejo basins was carried out to conduct a study of sedimentary provenance. The radiometric ages obtained by LA-ICP-MS show that most of the samples have common age spectra which are typical of North Gondwana sources: Archean and Paleoproterozoic ages characteristic of the West African craton, Neoproterozoic ages associated with Cadomian and Pan-African orogenies, and Cambrian ages related to the intra-continental rifting of North Gondwana. The only exception is a Visean immature greywacke that was probably derived from Devonian magmatic arcs related to the Variscan orogeny. Very rare pre-Devonian zircons indicate faint contributions from recycled sediments. In contrast the Serpukhovian to Moscovian greywackes are derived from felsic mature source rocks and include Proterozoic and Paleozoic detrital zircons suggesting recycling of an old basement. They also include Ordovician and Silurian detrital zircons, the ages of which have no correspondence in North Gondwana, and thus indicate an external source (Laurussia?). U-Pb ages younger than Ordovician are significant in the Triassic sandstone of the Alentejo basin and resemble the zircon populations of the Serpukhovian to Moscovian greywackes from the South Portuguese Zone. U-Pb ages younger than Neoproterozoic are poorly represented in the zircon population of the Triassic sandstone from the Algarve basin, which rests unconformably upon Moscovian turbidites, pointing to Upper Devonian quartzites of the South Portuguese Zone as probable sources, since they are dominated by Precambrian detrital zircons. These differences occurring in the Triassic basins suggest that detrital zircon populations could be derived from two independent sources and paleo-drainage systems due to complex crustal-block extension architecture

Variability over time in the sources of South Portuguese Zone turbidites: evidence of denudation of different crustal blocks during the assembly of Pangea

This study combines geochemical and geochronological data in order to decipher the provenance of Carboniferous turbidites from the South Portuguese Zone (SW Iberia). Major and trace elements of 25 samples of graywackes and mudstones from the Mértola (Visean), Mira (Serpukhovian), and Brejeira (Moscovian) Formations were analyzed, and 363 U-Pb ages were obtained on detrital zircons from five samples of graywackes from the Mira and Brejeira Formations using LA-ICPMS. The results indicate that turbiditic sedimentation during the Carboniferous was marked by variability in the sources, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (Mértola Formation and the base of the Mira Formation) inherited from a terrane with intermediate to mafic source rocks. These source rocks were probably formed in relation to Devonian magmatic arcs poorly influenced by sedimentary recycling, as indicated by the almost total absence of pre-Devonian zircons typical of the Gondwana and/or Laurussia basements. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (Mira and Brejeira Formations) included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism. The abundance of Precambrian and Paleozoic zircons reveals strong recycling of the Gondwana and/or Laurussia basements. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (Cadomian-Avalonian and Pan-African zircon-forming events), Paleoproterozoic, and Archean ages. The presence of late Ordovician and Silurian detrital zircons in Brejeira turbidites, which have no correspondence in the Gondwana basement of SW Iberia, indicates Laurussia as their most probable source

Levantamentos geofísicos no setor português da Faixa Piritosa Ibérica: perspetiva global da sua importância em prospeção de sulfuretos maciços e interpretação geológica

ABSTRACT: The application of geophysical techniques to massive sulphide deposits exploration has proven to be a success in the Iberian Pyrite Belt (IPB) metallogenetic province, both in Portugal and in Spain. Several hidden deposits were discovered through the interpretation of geological models, supported by geophysical data collected in land and airborne surveys, as well as applied using down hole techniques in selected drill holes. This paper presents an overview of the methodologies applied in Portugal in the South Portuguese Zone (SPZ) geological domain, where the IPB is included, mostly by the LNEG and mining companies, with emphasis on the interpretation of the regional gravity, magnetometry and radiometry surveys, published in 1/400 000 scale by the EXPLORA/Alentejo2020 Project. In this regional mapping program, the large volume of data collected by LNEG and mining companies since the 1960’s was compiled and processed. The integrated interpretation and correlation between these methods will enable a better planning of new IPB exploration investments. IPB Volcano-Sedimentary Complex (VSC, Famennian to Late Visean age) lineaments are well defined in the gravity and magnetic fields along the IPB western region (e.g. Alcoutim/Neves Corvo/Montinho/Sesmarias/Lousal/Caveira). Aeromagnetic data also reflect deep and large anomalies related with the SPZ basement. A rock density database is presented and used to interpret the gravity data. Radiometric map (Natural Radioactivity), based mostly in airborne radiometric surveys, show contrasts between rocks in Paleozoic, Mesozoic and Cenozoic formations and correlations with geological and hydrothermal mapping are highlighted. The use of geophysical techniques is particularly critical in areas where VSC is concealed by younger sediments (e.g. Baixo Alentejo Flysch Group and/or Cenozoic formations), allowing the selection of target areas. Case studies are discussed based on electromagnetic, seismic and magnetotelluric methods and on the high mineral potential of defined targets. Multilayered data correlation (geophysics including petrophysical data, geochemistry and stratigraphic high-resolution models) is a key prospecting methodology and is essential for the accurate modelling of ore horizons and geological structures. Considering the IPB potential for VMS deposits, including the possible discovery of new giant (> 200 Mt) deposits, an increase of investment in new geophysical methods can be predicted. LNEG development of R&D projects such as EXPLORA/Alentejo2020 and SmartExplorationTM/H2020 also contribute to a favourable setting in the province, both in near mining and green fields scenarios. Thus, new discoveries are expected, like others made in the recent times in the IPB Portuguese sector, such as Lagoa Salgada (SFM, 1992, SFM; Redcorp/EDM, 2011; Redcorp/Ascendant/EDM, 2017), Semblana (Somincor-AGC-Lundin Mining, 2010), Monte Branco (Somincor Lundin Mining, 2012) and Sesmarias (Maepa/Avrupa, 2014). The importance of these data goes beyond the mineral exploration purpose. Since some of them use rock and mineral physical properties that can be related with environmental contamination (e.g. radiometry and magnetics) they can also be used in geochemical background studies, acid mine drainage and metal dispersion. Therefore, they can also promote a deeper knowledge in other areas of research related with environment preservation.RESUMO: A aplicação de técnicas de prospeção geofísica na pesquisa de jazigos de sulfuretos maciços na Faixa Piritosa Ibérica (FPI) tem-se revelado um sucesso em Portugal e em Espanha. Vários jazigos ocultos foram descobertos a partir da interpretação de modelos geológicos, com o apoio de dados geofísicos recolhidos em levantamentos terrestres, aerotransportados ou em profundidade, em sondagens (diagrafias). Neste artigo, apresenta-se uma análise das metodologias aplicadas principalmente pelo LNEG e empresas mineiras em Portugal, na Zona Sul Portuguesa (ZSP), onde se insere a FPI. Destaca-se a interpretação dos levantamentos regionais, esc. 1/400 000, de gravimetria, magnetometria e radiometria (EXPLORA/Alentejo2020). Nestes programas, foi feito o processamento de um volume de informação significativo, a partir de dados obtidos desde a década de 1960. Neste contexto indicam-se diversos alinhamentos de gravimetria e de aeromagnetometria relativos a unidades geológicas com elevado potencial mineiro, como o Complexo Vulcano-Sedimentar (CVS, idade Famenniano a Viseano sup.), sobressaindo o eixo Alcoutim/Neves Corvo/Montinho/Sesmarias/Lousal/Caveira. Os dados de aeromagnetismo refletem também anomalias com origens mais profundas e relacionadas com a estrutura basal da ZSP. A base de dados de densidades de rocha é utilizada na interpretação gravimétrica da FPI. O mapa radiométrico (Radioatividade Natural), baseado fundamentalmente em radiometria aeroportada (contagens totais) mostra o contraste à superfície de litologias das formações paleozoicas, mesozoicas e cenozoicas, sendo salientado, no primeiro caso, a sua correlação com a cartografia de alterações hidrotermais. Em áreas onde o CVS se encontra subjacente sob sedimentos mais recentes (ex. Grupo Flysch Baixo Alentejo e formações cenozoicas), o uso de técnicas geofísicas é fundamental para a seleção de alvos. São também descritos exemplos sobre a aplicação de métodos eletromagnéticos, sísmicos e magnetotelúricos na identificação e caracterização de estruturas geológicas. A análise integrada de diferentes métodos geofísicos, complementados com informação geológica e geoquímica, constitui a melhor metodologia de prospeção, sendo essencial para a pesquisa de horizontes de mineralização. Considerando a FPI como uma das principais províncias metalogenéticas do mundo, é expectável que venham a ocorrer mais investimentos e assim serem identificados novos jazigos, inclusive com dimensão superior a 200 Mt. Através de projetos ID como o EXPLORA/Alentejo2020 e SmartExplorationTM/H2020 o LNEG contribui para um melhor conhecimento na FPI em cenários de prospeção near mining e green fields. Nesta perspetiva, são esperadas novas descobertas de jazigos à semelhança do que tem acontecido em Portugal, como por exemplo Lagoa Salgada (SFM-1992, Redcorp/EDM 2011, Redcorp/Ascendant/EDM-2017), Semblana (Somincor-AGC Lundin Mining, 2010), Monte Branco (Somincor-Lundin Mining, 2012) e Sesmarias (Maepa/Avrupa, 2014). A importância destes dados vai muito para além da prospeção mineira. Uma vez que alguns métodos utilizam propriedades físicas que podem ser relacionadas com contaminações ambientais. Certos métodos geofísicos como radiometria e magnetometria, resistividade e métodos elétricos em geral, podem ser utilizados também em trabalhos de investigação sobre background geoquímico, drenagem ácida de mina e dispersão de metais. Desta forma, a aplicação destes métodos promove também um conhecimento mais aprofundado noutras áreas de investigação relacionadas com a preservação do ambiente.info:eu-repo/semantics/publishedVersio

Geoquímica das rochas vulcânicas associadas aos jazigos de sulfuretos maciços do setor português da Faixa Piritosa Ibérica ocidental. Considerações sobre alteração hidrotermal, petrologia e evolução tectónica

ABSTRACT: A geochemical compilation database of the main volcanic units of the Volcano-Sedimentary Complex (VSC) of the Iberian Pyrite Belt (IPB), one of the largest provinces of massive sulfides on a global scale, is presented and discussed. For this purpose, we used several mineral exploration rock geochemistry databases from six IPB areas, namely Cercal, Lagoa Salgada, Lousal, Aljustrel, Neves Corvo and Chança, regarding unaltered/hydrothermaly altered felsic volcanic units interbedded in the Famennian-Late Visean volcano sedimentary sequences of the VSC. Volcanic rocks within the Phyllite-Quartzite Formation (Givetian-Famennian) IPB basement were also considered. From this, Neves-Corvo sector rhyolites (Rhyolite type 1 and 2) present the most intense hydrothermal alteration, directly related with the age of mineralization and coeval with hosted VSC sedimentary formations. Similar cases were also observed in the volcanic units hosting Lagoa Salgada, Chança and Aljustrel Volcanic-hosted massive sulfide (VHMS) deposits. The variety of felsic and intermediate volcanic rocks in the IPB (and within each sector) reflect different petrogenetic processes and/or distinct crustal sources. Application of Zr vs TiO2 binary diagrams allows to define three main trends of andesitic (Lagoa Salgada and Chança sectors), dacitic-rhyodacitic (Aljustrel sector) and rhyolitic (Cercal, Neves-Corvo, Aljustrel and Lousal sectors) composition. Cercal rhyolites are the most evolved felsic rocks (Zr/TiO2 ≈ 1562), followed by Neves-Corvo rhyolites (Zr/TiO2 ≈ 936), Lousal (Zr/TiO2 ≈ 845) and Aljustrel (Zr/TiO2 ≈ 840). In addition, four distinct compositional clusters can be distinguished based on Al 2O3/TiO2 vs Zr/TiO2 and Al2O3/Zr vs TiO2/Zr ratios according to its nature as, rhyolitic, rhyodacitic/dacitic and andesitic, probably reflecting differential partial melting rates. Ybn vs La/Yb(n) diagram ratio indicates that Neves-Corvo (mainly 2 types of rhyolites), Aljustrel (Tufo da Mina rhyolitic unit), as well as Lagoa Salgada sector rhyolites are projected along FIIIa and FIIIb rhyolite fields considered of higher metalliferous potential in the IPB, once their petrogenetic processes are considered ideal to trigger, sustain and host hydrothermal systems and consequently VHMS deposits. Future work, combining geochemical characterization of each volcanic unit with their stratigraphic positioning, is essential in order to achieve a correct correlation between the different sectors and, is therefore, a useful tool in IPB mineral exploration and drill-hole data correlation.Resumo: Neste trabalho são apresentadas e discutidas as interpretações resultantes de uma compilação de dados geoquímicos das principais unidades vulcânicas do Complexo Vulcano-Sedimentar (CVS) da Faixa Piritosa Ibérica (FPI), uma das maiores províncias de sulfuretos maciços à escala global. Para tal, foram utilizadas várias bases de dados de prospeção mineral relativas a análises químicas de rocha total para seis áreas: Cercal, Lagoa Salgada, Lousal, Aljustrel, Neves-Corvo e Chança (unidades vulcânicas intercaladas nas sequências vulcano-sedimentares de idade Famenniano-Viseano superior do CVS). Foram também considerados dados de rochas vulcânicas nos sedimentos basais da Formação Filito-Quartzítica (Givetiano-Famenniano). Os riólitos tipo 1 e 2 de Neves-Corvo apresentam a alteração hidrotermal mais intensa e estão diretamente relacionados com a idade da mineralização, sendo coevos com as formações sedimentares hospedeiras do CVS. Casos semelhantes são observados nas unidades vulcânicas que alojam os depósitos vulcânicos hospedeiros de sulfuretos maciços (VHMS) de Lagoa Salgada, Chança e Aljustrel. A variedade de rochas vulcânicas félsicas e intermédias na FPI (e dentro de cada setor) deve refletir diferentes processos petrogenéticos e/ou fontes crustais distintas. A aplicação do diagrama binário Zr vs TiO2 permite definir três tendências principais que correspondem aproximadamente às rochas de composição andesítica (setores da Lagoa Salgada e Chança), dacítica-riodacítica (setor de Aljustrel) e riolítica (setores do Cercal, Neves-Corvo, Aljustrel e Lousal). As rochas vulcânicas félsicas mais evoluídas são os riólitos do Cercal (Zr/TiO2 ≈ 1562), seguidos dos riólitos de Neves-Corvo (Zr/TiO2 ≈ 936), Lousal (Zr/TiO2 ≈ 845) e Aljustrel (Zr/TiO2 ≈ 840). Além disso, quatro grupos de composições distintas podem ser reconhecidos com base nas razões Al 2O3/TiO2 vs Zr/TiO2 e Al2O3/Zr vs TiO2/Zr de acordo com a sua natureza, riolítica, riodacítica/dacitica e andesítica, provavelmente refletindo diferentes taxas de fusão parcial. A aplicação do diagrama Ybn vs La/Yb(n) indica que os riólitos do setor de Neves-Corvo (principalmente dois tipos), Aljustrel (unidade riolítica Tufo da Mina) bem como as rochas vulcânicas da Lagoa Salgada são projetadas ao longo do campo dos riólitos do tipo FIIIa e FIIIb, sendo estes considerados os de maior potencial metalífero na FPI, pois os seus processos petrogenéticos são considerados ideais para desencadear, sustentar e hospedar sistemas hidrotermais e, consequentemente, depósitos de VHMS. Pretende-se, no futuro, continuar a desenvolver esta investigação envolvendo a caracterização geoquímica de cada unidade vulcânica, de acordo com o seu posicionamento estratigráfico, de forma a obter uma correlação sustentada entre os diferentes setores. Esta metodologia é assim uma ferramenta útil em prospeção mineral e na correlação de dados de diferentes sondagens.info:eu-repo/semantics/publishedVersio

Utilização do teste Kolmogorov-Smirnov para estudos de provenienciar sedimentar

O teste estatístico Kolmogorov-Smirnov (K-S) utilizado com dados de geocronologia de zircão foi aplicado a um estudo de proveniência sedimentar de diferentes litologias existentes na zona do Cabo de Sines (praias e arribas litorais), com o objetivo de testar as suas capacidades discriminativas, tendo permitido: i) definir a contribuição sedimentar das rochas que constituem as arribas; ii) realçar a existência de diferentes ciclos sedimentares; e iii) destacar diferenças entre a areia holocénica das praias e as rochas sedimentares e ígneas que constituem as arribas, cuja origem pode ser justificada por outras fontes ainda não identificadas

Sedimentary provenance of Neogene strata from the SW Portuguese coast (Sines Cape): detrital zircon U-Pb geochronology

In this preliminary study of provenance analysis Neogene sandstones and conglomerates of the Alvalade Basin were sampled, from the sea cliffs nearby the Sines Cape (SW Portuguese Coast). Detrital zircons were extracted by conventional methods of particle size separation, magnetic and heavy liquids separation. Detrital zircons were dated by U-Pb method with LA-ICPMS. The U-Pb geochronology results indicate as potential sources for all samples: i) the Carboniferous greywackes of the Mira Formation (South Portuguese Zone) or the upper Triassic sandstones of the Silves Formation Sandstones (Alentejo Basin) for the zircon ages older than the Permian; and ii) the sienites from the Sines Massif for the upper Cretaceous zircon ages. Note also that one sample includes a significant population of detrital zircon age of Permian age whose potential source is not known in the surrounding of the Alvalade Basin