Freixo de Numao Au-W deposit, Northern Portugal: ore features and mineralization controls

Abstract

The Freixo de Numao Au-W deposit is located in Vila Nova de Foz Coe region, northern Portugal. Mineralization is hosted in greenschist-facies metasedimentary rocks with lower-Cambrian age. The mineralized structures are subvertical and trend N10 degrees - 40 degrees E, crossing-cut bedding and/or regional foliation, and are post-peak of regional metamorphism. Quartz-sulfide veins and visible hydrothermal alteration characterize this mineralization in the field. The quartz veins record two main stages of mineral deposition (I and II), indicating repeat vein re-opening. Stage-I is characterized by milky quartz and arsenopyrite (Apy-1) and scheelite. Stage-II comprises clear quartz, adularia, chlorite, tourmaline, and metallic minerals, mainly arsenopyrite (Apy-2), chalcopyrite and pyrite, with galena and sphalerite in lesser amounts. The gold (native-Au and electrum) is associated with maldonite, native bismuth, and few Au-Ag phases. A detailed ore study has shown two different gold mode of occurrence: i) native-Au and maldonite, along with Bi-native as droplets in arsenopyrite and chalcopyrite; ii) electrum and Au-Ag phases infill interstitial cavities between arsenopyrite crystals and in its microfractures. These results match with some characteristics of other gold deposits in the European Variscan belt, which revealed a major crustal-scale hydrothermal event responsible by gold mineralization.