U–Pb LA-ICP-MS and Re–Os dating of wolframite and molybdenite: Constraints on multiple mineralization and cooling history (40Ar/39Ar) for the magmatic–hydrothermal system at Borralha, northern Portugal

Abstract

The Borralha area (Iberian Massif, Portugal) reflects a long-lived and multiphase magmatism (ca. 316 to 300 Ma), characterized by three types of granite emplacement during two different extensional events (E1 and E2) and one compressional event (C3) within the Variscan orogeny. The W (Cu) mineralization styles correspond to veinlets and quartz veins (i.e., São António and Santa Helena), and disseminations in the Santa Helena breccia pipe. Molybdenite coexists with an Fe-bearing wolframite–quartz assemblage in the Venise breccia pipe and quartz veins (i.e., São José and São Paulo).The geochemistry and geochronology of wolframite and molybdenite samples were studied by electron probe microanalysis (EPMA), in-situ U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and Re–Os thermal ionization mass spectrometry (TIMS). The wolframite yields a U–Pb age (sample WBV1) of 315.2 ± 4.7 Ma (2σ), representing the first recognized episode of W deposition in the Iberian Massif. Wolframite samples (WBV1, WB1, and WB4) have a ferberite composition with a hübnerite/ferberite ratio of < 29. High amounts of Nb (up to 3703 ppm) with low Ta (up to 123 ppm) and REEs (up to 11.4 ppm) and conversely, low amounts of Nb (up to 471 ppm) with high Ta (up to 190 ppm) and REEs (up to 53.7 ppm) were measured in wolframite samples. The REEs normalized patterns of the wolframite samples record a mixed signature. Large and positive Eu/Eu* and Er/Er* anomalies were identified (sample WBV1), whereas negative Ce/Ce* and Nd/Nd* anomalies were observed in samples WB1 and WB4.Molybdenite Re–Os dating yields absolute ages ranging from 305.1 to 303.8 ± 2.2 Ma (2σ), which are coeval with the emplacement of E2 biotite granite (Penedos; I-type; <305 Ma). Low amounts of Re (<1 ppm), W, Co, Ni, As, Cu, Bi, Fe, Mn V and Se (<0.005 wt%) were detected in the molybdenite structure.Constraints between the timing of W- and Mo-deposition and a protracted cooling history of the host rocks are confirmed by 40Ar/39Ar white mica dating. The 40Ar/39Ar ages obtained for secondary white micas collected from Mo veins (Venise breccia pipe; 277 ± 3 and 287 ± 4 Ma 2σ) and W (Cu) dissemination (Santa Helena breccia pipe; 287 ± 2 and 279.0 ± 6 Ma 2σ) record the isotopic resetting by later hydrothermal overprints and cooling /exhumation history after E2 Gerês biotitic granite emplacement.The W-ore deposition did occurred during the final E1 extensional tectonic stage (315 Ma) and E2 extensional tectonic stage for Mo-ore deposition, where a temporal offset was needed for the adjustment of tectonic conditions after the C3 compressional and E2 extensional stage started.