Stratiform Sb and Au mineralizations in the Hercynian Dúrico-Beirã area (North Portugal)

Preceeding the main Hercynian Au-As-Sb metallogenic hydrothermal event, metallic preconcentrations (Au, Sb, Pb) occurred during the sedimentation of the Paleozoic, namely in the Precambrian (?) and/or Cambrian layers bearing volcanic prints, in the volcano-sedimentary lower Ordovician and in the basal Carboniferous breccia. The most evident lithostratigraphic control in the Dúrico-Beirêo mining district concerns the Au-As mineralization hosted by the Arenigian alternances along the Eastern limb of the Valongo Anticline, with important syn-diagenetic concentration of gold related to exhalative “black-layer”, with pyrite-arsenopyrite. Lithostratigraphic controls are not so evident for the Sb-Au mineralizations that occur west of the Western limb of the Valongo Anticline. Nevertheless, all the studied deposits are hosted by competent sequences, mainly conglomerates, arenites and pelites from the Precambrian (?) and/or Cambrian (Montalto Unit) with acid volcanic rocks intercalated in the base, and in some cases by the Carboniferous basal breccia (paleo-placertype).