Fluid inclusion and (S, C, O, Pb) isotope study of Pb-Zn-(Cu-Ag) hydrothermal veins from Central and Northern Portugal – Metallogenic implications

Abstract

The Pb-Zn-(Cu-Ag) hydrothermal veins from Central and Northern Portugal occur in two distinct districts: Aveiro (Moinho da Pena, Carvalhal, Palhal and Telhadela) and Bragança (Ferronho, Vale da Madre, Estevais and Olgas). The ore veins are composed by sulfides (galena, sphalerite, chalcopyrite, pyrite), several quartz generations in both districts and dolomite as a gangue mineral in the Aveiro district (Palhal and Telhadela). A mineralogical, fluid inclusion and (S, C, O, Pb) isotopic study was carried out on ore mineralization with the aim to explain its genesis and metallogenic evolution.Aqueous fluids of H2O-CaCl2 or H2O-NaCl-CaCl2 in composition were identified in both districts. The Th/Tmi evolution starts with fluids of medium temperature (>250 °C) and low to moderate salinity (1 eq. wt% NaCl in the Aveiro district and 0.5 eq. wt% in the Bragança district), evolving to lower temperatures (250–100 °C) and higher salinity for the Aveiro district (20 eq. wt% NaCl) and lower salinity (~4 eq. wt% NaCl) for the Bragança district. Fluids of high salinity (~20 eq. wt% NaCl) were trapped during the final stage at temperatures below to 100 °C.A mixing magmatic and meteoric fluid explained the wide negative values range of δ34S measured for the samples from the Aveiro district. High positive δ34S values measured on samples collected from the Bragança district confirm a seawater source. The C and O stable isotope measured on dolomite and calcite show a 13C-depleted source for both carbonates, pointing to a carbon sources related to the organic matter oxidation. The Pb isotope data confirm an uranogenic and a thoriogenic source.The Pb-Zn ore deposits from the Aveiro and Bragança districts are related to episodes of fluid circulation in the upper crust during late to post-Variscan period. The lack of spatial and temporal relationship with magmatic episodes and the presence of basinal and superficial origin fluids support this idea.