The link between rhyolitic volcanism and the Aljustrel VHMS deposit Iberian Pyrite Belt: volcanic facies, geochemistry and geochronology

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Publications

Cadomian/Pan-African consolidation of the Iberian Massif assessed by its detrital and inherited zircon populations: is the similar to 610Ma age peak a persistent Cadomian magmatic inheritance or the key to unravel its Pan-African basement?

Publication . Chichorro, Martim; Solá, A. Rita; Santos, Telmo M. Bento Dos; Amaral, Joao Lains

ABSTRACT: This work assessed the age distribution of Cadomian/Pan-African orogenic events (550-590 and 605-790Ma, respectively) in several zones of Iberian Massif by means of detrital and inherited zircon analysis compilation. Detrital zircon age spectra show that throughout the late Neoproterozoic-to-Early Ordovician era (similar to 120Ma sedimentary record), the main systematic peak occurs at similar to 610Ma, followed by peaks at typical Cadomian ages (similar to 590-550Ma). Inherited zircons incorporated in Cambrian-to-Lower Ordovician igneous rocks show typical Cadomian ages (similar to 590-550Ma) but, once again, a remarkably consistent Pan-African similar to 610Ma peak occurs. In accordance with compiled zircon data and taking into account the evidence of North African peri-cratonic inliers, Ediacaran (similar to 610Ma) zircons incorporated in Paleozoic magmas provide indirect evidence of Pan-African magmatism, suggesting that these magmas and synorogenic sediments are likely to constitute the cryptic stratigraphic infrastructure of most of the Iberian Massif. The main source of similar to 610Ma inherited zircons may be the lateral chrono-equivalents of the Saghro and Bou Salda-M'Gouna Groups (Anti-Atlas, Morocco) and/or coeval igneous rocks from West African Craton or Trans-Sahara Belt, emplaced at a stratigraphic level below the late-Ediacaran sediments of the Ossa Morena Zone and the Central Iberian Zone. Assuming that the Iberian crust is a fragment of the Pan-African orogen, a relative paleoposition situated between the West African Craton and the Trans-Saharan Belt during the Late Neoproterozoic is proposed. The closed-system behaviour of Stenian-Tonian detrital zircon ages in the Trans-Sahara Belt suggests that this mega-cordillera acted as a barrier, in paleogeographic terms, to separating the Sahara Metacraton from Iberia. In Iberia, the opening of the system to Stenian-Tonian detrital zircon during the Ordovician indicates that, at that time, the Trans-Saharan Belt had already become a vast peneplain, which favoured a large drainage system with a long-distance transport mechanism that fed the passive continental margins.

2022-11Journal article

Open access

Detrital zircon similarities and dissimilarities between the Iberian Pyrite Belt, Ossa-Morena Zone and Meguma

Publication . Amaral, Joao Lains; Solá, A. Rita; Santos, Telmo M. Bento Dos; Chichorro, Martim

ABSTRACT: Despite the so-called exotic nature of the South Portuguese Zone relatively to the other major domains of the Iberian Massif of peri-Gondwanan affinity, Devonian detrital rocks of the oldest strata in the Iberian Pyrite Belt have a remarkable resemblance with the Ossa-Morena Zone’s Neoproterozoic-Cambrian rocks and the West Meguma’s Cambrian-Ordovician rocks, presenting the so-called “West African signature”.Using published U-Pb detrital zircon data, we discuss the similarities and dissimilarities between the Iberian Pyrite Belt, Ossa-Morena Zone and West Meguma Terrane through multidimensional scaling, comparing them with other zones of the Iberian Massif, Saxo-Thuringian Zone, Avalonia-Ganderia, and the North African cratonic regions. Our findings show that multidimensional scaling is not entirely effective in displaying the dissimilarities between the peri-Gondwanan terranes due to the background noise caused by the overwhelming number of Cadomian-Panafrican ages. However, it becomes a powerful tool if these ages are filtered. A dominant Meguma-type provenance (Cambro-Ordovician) for the Middle-Upper Devonian rocks of the Iberian Pyrite Belt is demonstrated, mainly attending to their similar Birimian-Eburnean pattern. The possibility of minor contributions from the lower Cambrian rocks of the Ossa-Morena Zone into the Iberian Pyrite Belt quartzites is unlikely, as the latter lack the 1.9Ga peak that characterises the Ossa-Morena Zone sediments. Additionally, the remarkable similarities between Ossa-Morena Zone’s and West Meguma’s detrital rocks strongly suggest a similar paleogeographic setting (but diachronic) for both terrains from the Ediacaran to Lower Ordovician times relative to the North African blocks.

2022-12Journal article

Open access

The bimodal Fii-A2-type and calc-alkaline volcanic sequence of the Aljustrel brownfield region, Iberian Pyrite Belt, SW Iberian Massif

Publication . 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, João

ABSTRACT: 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.

2024-04Journal article

Open access

Funding agency

Fundação para a Ciência e a Tecnologia

Funding programme

OE

Funding Award Number

SFRH/BD/138791/2018