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

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 (Gavião, São João, Moinho, Algares, Estação and Feitais). To delve into the petrogenesis of the Aljustrel early Carboniferous (∼355 Ma) felsic-dominated bimodal volcanism, new whole-rock trace elements and SmNd isotopes, and UPb 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. UPb 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 (ƐNdi = +1.79) and evolved felsic rocks (ƐNdi = −5.07), imply heterogeneous sources dominated by zircon-bearing igneous rocks. The SmNd model ages are in accordance with previous LuHf 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 SmNd isotopic data (ƐNdi = +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.