Antimony quartz and antimony-gold quartz veins from northern Portugal

Abstract

Antimony- and Pb-Sb-quartz veins from the Bragança district, Portugal, are mainly hosted by Silurian phyllites. Antimony-Au-quartz veins from the Dúrico-Beirã region are mainly hosted by a Cambrian schist-metagraywacke complex, as well as Ordovician phyllites and quartzites. The deposits were mostly exploited in the late 19th Century. Mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas. First and second generations of arsenopyrite precipitated at 390 and 300 °C, respectively. Berthierite and stibnite are the most abundant Sb-bearing minerals and precipitated between 225 and 128 °C, native antimony at < 200 °C. Drastic fluid cooling is the main cause of mineral precipitation. The Pb isotope compositions of stibnite suggest a homogeneous crustal source of lead, from the metasedimentary sequences, for Sb, Pb-Sb and Sb-Au deposits in both areas, which is consistent with the findings for comparable mineralizations elsewhere in Europe. Remobilization of Pb is related to Variscan metamorphism and deformation.