U-Pb zircon SHRIMP dating of a protracted magmatic setting and its volcanic emplacement: Insights from the felsic volcanic rocks hosting the sulphide ore of the giant Aljustrel deposit, Iberian Pyrite Belt

Abstract

• A protracted (~50 Ma) magmatic activity is suggested. • Upper Devonian subvolcanic activity (~373–365 Ma) took place before main volcanism . • Early Carboniferous long-lasting volcanism was identified (353–359 Ma) • Data suggests diachronism of sulphide deposition between different sub-basins. • Pb loss effects are attributed to the Variscan deformation and metamorphism . A 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.